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AN 

ELEMENTARY TREATISE 

ON 

PHOTOTOPOGRAPHIC METHODS 

AND INSTRUMENTS 

INCLUDING A CONCISE REVIEW OF EXECUTED 

PHOTOTOPOGRAEHIC SURVEYS AND OF 

PUBLICATIONS ON THIS SUBJECT 



J. A. ELEMER 

Topographical Engineer 



FIRST EDITION 
FIRST THOUSAND 



NEW YORK 

JOHN WILEY & SONS 

London: CHAPMAN & HALL, Limited 

1906 



A ■2-a%(:>%\ 
Copyright, igo6 

BY 

J. A. FLEMER 



KOEEET DRUMMOND, PRINTER, NEW YORK 



PREFACE. 



Light-rays, in addition to their heating and illuminating 
qualities, have a chemical or so-called " actinic " power, char- 
acterized by a general tendency to decompose certain chemical 
compounds, some of which, for that very reason, being utilized 
in photography. 

The term photography, composed of two Greek words 
phos, or phota, and grapho, means light-drawing or sun- 
picture, and a photograph may be defined as a picture pro- 
duced by the actinic action of light-rays (emanating from the 
object to be pictured) upon a surface chemically prepared for 
this particular purpose. 

Although the origin of photography may be ascribed to the 
alchemists of the sixteenth century, who had observed that chlo- 
ride of silver becomes blackened when subjected sufficiently 
long to the action of light-rays, still, photography as we know 
that art to-day is not so very old. 

In 1777 the Swedish chemist Scheele discovered that the 
intensity of the actinic power of light-rays changed with the 
quality of the light, inasmuch as argentic chloride would turn 
black quicker when exposed to the rays of the violet end of the 
solar spectrum. 

Wedgewood and Davy probably produced the first " sun- 
pictures" obtained by the action of light-rays upon surfaces 



IV PREFACE. 

sensitized with nitrate of silver. Their pictures, however, were 
not permanent — they were not " fixed." 

Joseph Nicephore Niepce produced the first permanent sun- 
pictures in 1 814 by a process known under the name of heliog- 
raphy, and in 1824 Daguerre commenced his researches and 
experiments which eventually (after Niepce and Daguerre had 
formed a copartnership in 1829) 1^^ t° ^^^ perfection of the 
so-called " daguerreotype." 

In 1839, while alluding to the details of the daguerreotype 
process before the Chamber of Deputies, in Paris, Arago declared : 
..." Nous pourrions, par exemple, parler de quelques idees 
qu'on a eu sur les moyens rapides d'investigation, que le topo- 
graphe pourra emprunter a la photographic." . . . 

Gay Lussac expressed himself in a similar manner when 
he had occasion to refer to the possibilities in adapting photog- 
raphy to topographical surveys. At about the same period, or 
possibly a little earlier. Fox Talbot read a paper on photogenic 
drawings before the Royal Society in London, describing his 
method for obtaining silhouettes, or shadowgraphs, of objects 
on paper that first had been treated with a solution of common 
table salt, then dried and immersed in a solution of nitrate of 
silver. The salt absorbed by the paper converts a part of the 
nitrate of silVer deposit into chloride of silver, some of the silver 
nitrate remaining unaffected in the paper. Talbot fixed these 
silhouettes on the paper by treating the outline pictures with a 
solution of bromide of potassium. 

In 1 841 Talbot had perfected his method to reproduce objects 
in general, and his method thereafter became known as the 
Calotype or Talbotype. 

In 1 85 1 Scott Archer introduced the so-called " wet collo- 
dion process," which remained in general use during the fol- 
lowing twenty-five years. The wet collodion process required 
the plates to be coated immediately before use, first with collo- 
dion iodide and again with the sensitizing silver bath, and imme- 
diately after exposure they had to be developed and fixed. For 



PREFACE. 



outdoor photography a dark-tent or a dark-room wagon had to 
be provided and the necessary chemicals had to be carried along 
in the field, together with the camera, plates, and plate-holders. 
The old photographic cameras, moreover, were unwieldy and 
cumbersome, and to apply photography on exploring expeditions 
and travels generally required such an increase in baggage, 
not to mention the need of special expert knowledge in photo- 
graphic chemistry, that amateur photography was practically 
unknown during the period when the wet-plate process flourished. 

All these difficulties, restricting the practice of photography 
to professionals, were overcome when, in 1871, Dr. Maddox 
discovered the so-called " dry-plate process " and published the 
details of his " gelatine-emulsion dry-plate coating " for photo- 
graphic plates. 

Dr. Maddox had succeeded in preparing an emulsion of 
bromide of silver in gelatine, with which plates could be coated 
and dried in the dark-room until the film had become quite 
hard, such plates remaining sensitive to the action of light-rays 
long after the sensitized film had been applied. The present 
general popularity of photography and its extensive application 
in various branches of the sciences as well as in the arts dates 
from Dr. Maddox's invention, which, in 1873, was improved by 
Kennett. 

Collodion has now been entirely replaced by gelatine for all 
outdoor work and gelatino-bromide dry-plates are manufactured 
in large quantities and in such variety as to answer all demands 
and requirements of the different applications of photography 
of the present day. 

The method of photographic surveying as developed by 
Col. A. Laussedat, who in his first experimental work used the 
" camera clara," now became more widely known and Lausse- 
dat' s methods found practical application in several countries, 
where they now receive a general recognition as valuable adjuncts 
to the instrumental topographic methods. 

Notwithstanding the recent rise in favor of photography. 



vi PREFACE. 

applied to the sciences in general and to topographic surveys in 
particular, we still meet with many surveyors who seriously 
question the practical value and general accuracy of photographic 
surveys, either from misconception of their guiding principles, 
from defective results so often obtained from a mere mechanical 
appHcation of phototopographic rules and methods, or more fre- 
quently still, from extreme conservatism. 

Others, again, may have failed to become interested in photo- 
graphic surveying, believing that a thorough familiarity with 
the theories and laws of optics, photochemistry, descriptive 
geometry, perspective and general cartography are essentials 
without which no practical knowledge and understanding of 
photographic surveying may be obtained. 

It should, of course, be admitted that such knowledge will 
enable the student to master phototopography in a rapid and 
easy manner, giving him an unquestioned advantage in, and an 
enlarged field for, the practical application of photography to 
surveying, or in teaching photogrammetric methods to others, 
yet the fundamental principles underlying these methods are so 
simple that it is believed any topographer (with the knowledge 
that he should have as such) may readily acquire enough of the 
theory to become fully able to apply photography quite success- 
fully to practical surveying, especially if he is famiUar with the 
methods of the plane table. 

This treatise has been written primarily with a view towards 
overcoming some of the existing prejudices against photographic 
surveying, and if it aids to demonstrate that this branch of 
surveying may rightly be assigned a legitimate place in the cur- 
riculum of every modern topographer, filling as it does a par- 
ticular gap in the general series of topographic methods here- 
tofore recognized, its existence will be justified. 

In the following pages reference will be made chiefly to 
those photogrammetric methods which find application to topo- 
graphic surveys, although the same principles are also used when 
applying photography to: 



PREFACE. VU 

Geological Surveys, made for the study of volcanic eruptions 
and their effects; for the study of recurrent changes in sand- 
dunes caused by winds blowing from certain directions at regu- 
lar intervals; for noting changes in glaciers (glacial motion or 
variation), based upon the comparison of glacier maps and 
photographs obtained at stated time intervals from identical and 
fixed camera stations, etc. 

Meteorological Observations, for the study of the higher air- 
currents and cloud altitudes, based upon iconometric cloud 
charts obtained from photographic plates exposed at two or 
more stations (simultaneously) at stated time intervals; for the 
study of the paths of lightning, their lengths, forms, etc. 

Hydrographic Surveys, for the location of rocks, buoys, cur- 
rent floats, etc. ; for the study of fluvial currents, riparian changes 
due to corrasion, erosion, shoaling, or silting, etc.; for obtaining 
coast views from points marked on the charts to serve for future 
determinations of the positions of vessels that may sight the 
land from the same locality in regions where fogs prevail; for 
preliminary surveys of coastal belts while conducting a hydro- 
graphic survey of the coast, harbors, etc. 

Engineering Works, for estimating the amount of work accom- 
plished at any date, based on photographic records showing the 
status of the work at specified dates, excavations, cuts, fills, struc- 
tural buUdings, etc.; for preliminary surveys to be used for the 
location of roads, irrigation dams, canals, etc. 

Architectural Surveys, for constructing the ground plans and 
elevations of old buildings from their photographs; for purposes 
of renovation, remodeling, publication, etc. 

Military and Secret Surveys, for establishing the positions of 
the enemy's forces; for locating fortifications; for establishing 
range lines for artillery use; for obtaining topographic recon- 
naissance maps, etc. 

This treatise wiU indicate, in a general way, how photography 

,may be applied to topography by describing the simple processes 

and methods, particularly those of a graphic character, that will 



Vlll PREFACE. 

suffice to direct beginners in their practical application, leaving 
it to experience and subsequent special study to determine the 
measure of success, the more so as several excellent works on 
this subject have been published recently in the English, French, 
German, Spanish, and Italian languages. 

Preference should be given to the graphic solution of icono- 
metric problems, inasmuch as topographic maps are primarily 
graphical records of instrumental measurements made in the 
field for locating the principal points of the terrene. Artificial 
details and topographic features are largely sketched, or inter- 
polated, when using the ordinary topographic surveying methods, 
whereas this sketching may be reduced to a minimum when 
applying photogrammetric methods by determining an increased 
number of points that control the characteristic horizontal and 
vertical changes in the terrene, which, in this case, are located 
graphically upon the chart by means of iconometric transfers 
from the photographic perspectives of the, landscape. 

The main control for a phototopographic survey should, 
of course, be of trigonometric origin and the coordinates of the 
triangulation points should be computed with a degree of accu- 
racy commensurate with the degree of precision attained in the 
field observations. 

The writer, having consulted all available publications on 
phototopographic methods and instruments in use, gladly acknowl- 
edges his indebtedness for valuable information on this subject 
to Col. A. Laussedat, Director of the Conservatoire des Arts 
et Metiers, Paris; Capt. E. Deville, Surveyor-general of Dominion 
Lands, Ottawa, Canada; Dr. W. F. King, Alaskan Boundary 
Commissioner to H. M., Ottawa; SignoreL. P. Paganini, Engineer 
Geographer of the MiHtary Geographical Institute of Italy; 
Dr. R. Doergens, Prof, of Geodesy, Royal Technical High School, 
Charlottenburg, Prussia, and particularly to the following pub- 
lications : 

" La Fototopografia in Italia," L. P. Paganini, Rivista Marit- 
tima, 1889; 



PREFACE. IX 

" Nuovi Appunti di Fototopografia," L. P. Paganini, Riv, 
Marittima, 1894; 

" Zeitschrift fuer Vermessungswesen," Stuttgart; 

" Zeitschrift fuer Instrumentenkunde " ; 

" Lehrbuch der Praktischen Photographie," Dr. A. Miethe; 

"Anthony's Photographic Journal"; 

" Die Photographische Messkunst," Prof. F. Schiffner, 
Halle a./S., 1892; 

" Die Anwendung der Photographie in der Practischen Mess- 
kunst," E. Dolezal, Halle a./S., W. Knapp, 1896; 

" Die Stereo-Photogrammetrie," MaJ. von Huebl, Mitth. 
des k. u., k. Militaer-Geographischen Inst., XII, 1903; 

" Photographic Surveying," E. Deville, Ottawa, 1895; 

" Photogrammetrie und Internationale Wolkenmessung," Dr. 
C. Koppe, Braunschweig, 1896; 

" Comptes Rendues de I'Academie des Sciences," Paris; 

" Annales de I'Observatoire Metgorologique du Mont Blanc," 
J. Vallot, Paris, 1896; 

" Recherches sur les Instruments, les Mdthodes et le Dessin 
Topographiques," Col. A. Laussedat, Paris, 1898. 



CONTENTS. 



PAGE 

Introduction i 



CHAPTER I. 

SHORT REVIEW OF EXECUTED PHOTOGRAPHIC SURVEYS AND LISTS OP THE 
MORE IMPORTANT PUBLICATIONS ON PHOTOGRAMMETRY .AND PHOTO- 
TOPOGRAPHY. 

I. Photographic Surveying in France 5 

Literature (French). . . '. 9 

II. Photographic Surveying in Germany 12 

III. Photographic Surveying in Austria 15 

Literature (German and Austrian) 16 

IV. Photographic Surveying in Sweden 21 

Literature (Swedish) 22 

V. Photographic Surveying in Switzerland 23 

VI. Photographic purveying in Italy 25 

VII. Photogra,phic Surveying in Spain 27 

VIII. Photographic Surveying in the Dominion of Canada and in Alaska. 29 

Literature (English) 33 



CHAPTER II. 

THE ELEMENTS OF PERSPECTIVE (CENTRAL PROJECTION). 

I. Visual Seeing 35 

II. Central Projection 36 

III. Photographic Perspectives 36 

xi 



XU CONTENTS. 



CHAPTER rn. 

PIN-HOLE PHOTOGRAPHY. 

PAGE 

I. Diameter of the Pin-hole 42 

n. Length of Exposure 43 

ni. Focal Lengths of Pin-hole Cameras 44 

IV. Determination of the \'alues of the Pin-hole-camera Constants. . . 44 



CHAPTER IV. 

FUNDAMENTAL PMNCEPLES OF ICONOMETRIC MAP-PLOTTING ("ICONOMETEY"). 

I. Orienting the Picture Traces on the Plotting-sheet 47 

A. Iconometric Plotting when using a Sur\-eying Camera only . 49 

B. Plotting the Picture Traces when using a Camera or Photo- 

theodoUte 49 

n. Arithmetical Determination of the Principal and of the Horizon 

Line on the Photographic Perspectives 51 

A. Determination of the Principal Ppint and Distance Line of 

the Photographic Perspective 51 

B. Determination of the Position of the Horizon Line on the 

Perspective 5 

in. Graphic jMethod for Determining the Positions of the Principal 

and Horizon Lines on the Perspectives 54 

IV. The "Five-point Problem" (by Prof. F. Steiner), or Locating 
the Plotted Position of the Camera Station by means of the 
Perspective when Five Triangulation Points are Pictured on 
the Same Photographic Perspective 55 

A. Determination of the Principal Point and Distance Line .... 55 

B. Simplified Construction for Locating the Plotted Position 

of the Camera Station by Means of the "Five-point 
Problem " 56 

C. Application of the "Five-point Problem" to the Special Case, 

where the Five Points range themselves into a Triangle 

on the Working-sheet 57 

D. To Find the Elevation of the Camera Horizon for a Station 

that has been Located by Means of the "Five-point 
Problem" 58 



CONTENTS. xiii 

PAGE 

V. The "Three-point Problem" 59 

A. Mechanical Solution of the "Three-point Problem" (using a 

Three-arm Protractor or Station-pointer) 60 

B. Graphic Solution of the "Three-point Problem" 60 

1. Using the so-called "Two-circle Method" 60 

2. Using the Method of Bohnenberger and Bessel 61 

VI. The Orientation of Picture Traces, Based on Instrumental Measure- 
ments Made in the Field 62 

A'll. Relations between Two Perspectives of the Same Object, Viewed 

from Different Stations. (Prof. Guido Hauck's Method) . . 62 

A. "Kernel Points" and "Kernel Planes" 64 

B. Use of the "Perspective Axis" (Line of Intersection) of Two 

Picture Planes that show Identical Objects Viewed from 

Different Stations 66 

VIII. To Plot a Figure, Situated in a Horizontal Plane, on the Ground 

Plan by Means of its Perspective 68 

IX. To Draw the Horizontal Projection of a Plane Figure on the Ground 
Plan by Means of the so-called "Method of Squares," if its 
Perspective in Vertical Plane and the Elements of the Per- 
spective are given 72 

X. The "Vanishing Scale" 74 



CHAPTER V. 

PHOTOGRAPHS ON INCLINED PLATES. 

A. To Plot the Picture Trace of an Inclined Plate 78 

B. Plotting the Lines of Direction to Points Pictured on an Inclined 

Photographic Plate 79 

C. Determination of the Altitudes of Points Pictured on an Inclined 

Plate 80 

D. Applications of Prof. Guido Hauck's Method 80 

CHAPTER VL 

PHOTOTOPOGKAPHIC SURVEYING METHODS. 

I. Analytical or Arithmetical Phototopographic Methods 83 

A. Method of Prof. W. Jordan 83 

B. Method of Dr. G. Le Bon 86 



XIV CONTENTS. 

PAGE 

G. Method of L. P. Paganini (Italian Method) 88 

1. Determ'nation of the Focal Length of the Photographic 

Perspective 88 

a. When the Reference Point is Bisected by the Prm- 

cipal Line of the Perspective 88 

Example No. i 89 

h. The Image of the Reference Point falls to either 
Side o' the Principal Line of the Photographic 

Perspective 90 

Example No. 2 • • • ■ 91 

Example No. 3 92 

2. Orientation of the Picture Traces 92 

3. Determination of the Elevations of Pictured Terrene 

Points 95 

Example No. 4 95 

(a) Computation of the Focal Length (/) 98 

iff) Computation of the .Abscissae for Plotting 
Lines of Horizontal Directions to Pictured 
Points of the Terrene and for Checking 

the Position of the Principal Point 98 

(y) Computation of the O dinates of Pictured 
Terrene Points of Known Elevations to 
Check the Position of the Horizon Line on 

the Negative 98 

{S) Orienting a Panorama 99 

4. Checking the Verticality of an Exposed Plate 100 

Example No. 5 102 

5. Application of Franz Hafferl's Method for Finding the ' 

Focal Length of a Photographic Perspective from 

the Abscissas of Two Pictured Terrene Points. ... 107 

Example No. 6 108 

Example No. 7 iii 

6. Supplement 113 

a. Forms Showing Arrangements of Field Records for 

Panorama Views 113 

h. Form used for Recording the Elevations of Second- 
ary Points of the Panorama Views 115 

D. General Arithmetical Method for Finding the Plotted Posi- 

tions of Terrene Points when Pictured on Vertically Ex- 
posed Picture Planes lie 

E. General Arithmetical Method for Finding the Plotted Posi- 



CONTENTS. XV 

PAGB 

tions of Terrene Points when Pictured on Inclined 

Picture Planes 117 

F. General Arithmetical Determination of the Elements of a 

Photographic Perspective 119 

IL Graphical Iconometrical Plotting Methods 121 

A. Col. A. Laussedat's Method (French Method) 121 

1. Orientation of the Picture Traces on the Plotting-sheet. 124 

2. Locating Points on the Plotting-sheet that have been 

Identified on Several Photographs 124 

3. The Iconometric Determination of Elevations of Pic- 

tured Terrene Points 125 

4. Drawing the Plan Including Horizontal Contours 126 

B. Method of Dr. A. Meydenbaur (German Method) 128 

1. Determination of the Focal-length Value for the Photo- 

graphic Perspective 128 

2. Orientation of the Picture Traces on the Plotting-sheet. r30 

3. Locating Points, Identified on Several Photographs, on 

the Plotting-sheet 131 

4. The Iconometric Determination of Elevations of Pic- 

tured Terrene Points 132 

C. Capt. E. Deville's Method (Canadian Method) 132 

1. General Remarks on the Field-work 132 

2. General Remarks on the Iconometric Plotting of the 

Survey 135 

3. Orienting the Picture Traces on the Working-plan 137 

4. The Identification of Pictured Points in Photographs 

Representing Identical Points of the Terrene 138 

5. Application of Prof. G. Hauck's Method for the Iden- 

tification of Terrene Points Pictured on Several 
Photographs 139 

6. Plotting Pictured Terrene Points as Intersections of Lines 

of Horizontal Directions. Iconometric Plotting of 
Terrene Points in GeneraL ("Horizontal Inter- 
sections") r40 

7. Iconometric Plotting of Pictured Terrene Points by so- 

called " Vertical Intersections " 142 

8. Iconometric Determination of the Elevations of Pic- 

tured Terrene Points 145 

9. Iconometric Determination of the Elevations of Pic- 

tured Terrene Points by Means of the so-called 
"Scale of Heights" 147 



XVI CONTENTS. 

PAGE 

10. The Use of the so-called "Photograph Board" 148 

11. Iconometric Plotting of the Trace of a Figure's Plane. . 149 

12. Iconometric Contouring 151 

13. The Use of the so-called "Photograph Protractor" 154 

D. Method of Conunandant V. Legros for Locating the Horizon 

Line of a Vertically Exposed Plate 156 

E. Prof. S. Finsteiwalder's Method for the Iconometric Plotting 

of Horizontal Contours iS7 



CHAPTER Vn. 

CAMERA LENSES. 

A. The Refractive Index 159 

B. Refraction of Light-rays 160 

C. The Optical Lens 161 

D. Optical Distortion 163 

E. Nodal Points and Nodal Planes of a Lens 165 

F. Principal Foci and Focal Planes of a Lens 168 

G. The Focal Length of a Lens 169 

H. The Biconvex or Positive Lens 170 

I. Conjugate Foci and Conjugate Planes 171 

K. To Find the Image of any Luminous Point for the Biconvex Lens. . 172 

L. The Biconcave or Negative Lens 173 

M. To Find the Image of a Luminous Point for a Biconcave Lens 174 

N. Lens Combinations 174 

O. Diaphragms or Lens Stops 176 

P. Rapidity of a Lens 176 

Q. Length of Exposure 177 

R. Distortion Produced by Diaphragms 177 

S. Chromatic Aberration of Light-rays 178 

CHAPTER VIII. 

PHOTOGEAMMETERS OR PHOTOGRAMMETRIC INSTRUMENTS. 

General Requirements to be FulfiUed by a Topographic Surveying- 
camera J83 

I. Ordinary Cameras (with Extension Bellows) Converted into Sur- 
veying-cameras 184 

n. Special Surveying-cameras with Constant Focal Lengths 185 

A. Dr. A. Meydenbaur's New Small Magazine Camera 185 



/ 

CONTENTS. 



PAGE 



B. Capt. E. Deville's Surveying-camera (New Model) i86 

1. Determination of the Focal Length, the Horizon Line, 

and the Principal Line igo 

2. Adjustment of Camera Spirit-levels 192 

3. Use of the Instruments Comprised in the Canadian 

Phototopographic Outfit ig^ 

C. The U. S. Coast and Geodetic Survey Phototopographic 

Cameras jpg 

D. L. P. Paganini's New Phototopographic Instrument for 

Reconaissance Surveys on Scales of 1:50000 and 

1 : 100,000 (Model of 1897) 200 

1. The Phototopographic Camera Proper 201 

2. The Horizontal Graduated Circle 203 

3. The Azimuth Compass 20^ 

4. The Tripod ^ 204 

5. Adjustments and Use of the Instruments 204 

m. Surveying-cameras Combined with Geodetic Instniments. (Pho- 

totheodolites, Phototachymeters, Photographic Plane Tables, 
Etc.) 209 

A. L. P. Paganini's Photogrammetric Instrument (Model of 

1884) 211 

Constant Focal Lengths of the Italian Cameras 215 

B. L. P. Paganini's New Phototheodolite (Model of 1894) 219 

C. L. P. Paganini's Photographic Azimuth Compass (Photo- 

graphic "Azimutale' ) 223 

D. Photogrammetric Theodolite of Prof. S. Finsterwalder 227 

E. Phototheodolite for Precige Work, by O. Ney 231 

F. Phototheodolite of Dr. C. Koppe 234 

G. Dr. C. Koppe's New Instrument and Method for Observing 

Horizontal and Vertical Angles Directly on the Photo- 
graphic Negative 236 

H. Phototheodolite Devised by V. Pollack; Manufactured by 

R. Lechner in Vienna, Austria 241 

I. Phototheodolite Devised by Pollack and Hafferl 242 

K. R. Lechner's Photogrammeter 243 

L. Phototheodolite of Col. A. Laussedat (New Model) 244 

' M. Phototheodolite of Starke and Kammerer 245 

N. Capt. von Huebl's Plane-table Photogrammeter 250 

O. Phototheodolite ("Pbototacheomfetre") Devised by J. and 

H. Vallot 253 

P. Phototheodolite Designed by J. Bridges-Lee 262 



XVUl CONTENTS. 

CHAPTER IX. 

panorama cameras. 

pAge 

I. The Photographic Plane Table Devised by A. Chevallier 270 

II. The Rockwood-Shallenberger Panoramic Camera 271 

III. R. Moessard's Topographic CyUndrograph 271 

CHAPTER X. 

ICONOMETERS AND PERSPECTOGRAPHS. 

I. Graphic Protractor (of L. P. Paganini) '. . . 275 

II. L. P. Paganini's Graphic Sector ("Settore Grafico") 278 

III. L. P. Paganini's Graphic Hypsometer ("Squadro Grafico") 284 

IV. The Centro-linead as Used by Capt. E. Deville 290 

A. To Set the Arms / and /' of the Centro-linead if the Direc- 

tion to the Vanishing Point be given by a Line in the 
Ground Plan 293 

B. To Set the Arms of the Centro-linead if the given Line 

(VE) Belongs to the Perspective 294 

V. The Perspectometer as Used by Capt. E. Deville 294 

VI. The Perspectograph Devised by H. Ritter 298 

Use of the Perspectograph 301 

VII. Prof. G. Hauck's Trikolograph and its Use in Iconometric Plotting. 302 
VIII. The Carl Zeiss Stereoscopic Telemeter and the Stereocomparator, 
Including the Stereophotogrammetric Surveying Method De- 
vised by Dr. C. Pulfrich 308 

A. The Stereoscopic Telemeter, or Range-finder 309 

B. The Stereocomparator and the Stereophotogrammetric Sur- 

veying Method 318 

CHAPTER XI. 

PHOTOGRAPHIC OPERATIONS IN THE FIELD. 

I. General Remarks on the Exposure of a Photographic Dry Plate . . 335 

II. Orthochromatic Dry Plates and Ray-filters 339 

A. Color-screens, or Ray-filters 340 

B. Halation 342 

III. Comparative Light Values and Exposures 343 

Test Exposures and Trial Plates 346 



CONTENTS. XIX 

PAGE 

rV. Development of Orthochromatic Dry Plates 348 

A. Water and Water Tests 352 

B. Developers 353 

1. Developing with Ferrous Oxalate 553 

a. Restraining the Ferrous Oxalate Development. . . . 356 

6. Accelerating the Ferrous Oxalate Development. . . 357 

2. Pyro Developer 358 

3. Metol Developer 359 

4. Metol-bicarbonate Developer 360 

5. Hydrochinon Developer 361 

6. Metol-hydrochinon Developer 363 

7. Bromo-hydrochinon Developer 364 

8. Eikonogen Developer , 364 

9. "Eiko-cum-hydro" Developer 365 

10. Amidol Developer 366- 

C. Fixing the Negative 366' 

1. Tests for Presence of Hyposulphite of Soda 368. 

2. Drying the Finished Negative 369. 

3. Intensification of a Negative with the Aid of Metallic 

Salts 369 

4. Intensification with Silver Cyanide 370 

5. Prof. R. E. Liesegang's Intensifier 371 

6. Intensifying Negatives without the Use of Metallic Salts . 372 

7. Reducing the Density of a Negative 373 

8. Cooling Solutions 374 

D. Negative Varnish 375 

V. Photographic Printing 376 

A. Toning Photographic Prints 379 

B. Fixing Photographic Prints 381 

C. Formulas for Plain Toning-baths ■ 382 

D. Combined Toning- and Fixing-baths 385 



CHAPTER Xn. 

CONCLUSION AND REMARKS ON THE PRECISION OF THE "POLAR ICONOilETRIC 
method" AXD GENERAL REMARKS ON TELEPHOTOGRAPHY. 

I. General Remarks on Phototopography 387 

n. Precision of the "Polar-iconometric" Method 391 

m. General Remarks on Telephotography or Long-distance Pho- 
tography 402 



PHOTOTOPOGRAPHIC METHODS AND 

INSTRUMENTS. 



INTRODUCTION. 

Topography is that branch of surveying which pictures 
sections of the earth's surface, in reduced scale, as a horizontal 
projection, showing the relative positions of points of the ter- 
rene in both the horizontal and vertical sense. Under topog- 
raphy, in the closer sense, we generally understand the represen- 
tation of the terrene in the form of charts, dravra to the scales 
of 1 : 5000 to 1 : 1 00000, showing not only the relative positions 
of characteristic points of the earth's surface, but also clearly 
delineating all natural and cultural details. Topographic charts 
on scales smaller than i : looooo partake of a geographic char- 
acter, while surveys on scales larger than i : 5000 are generally 
made for special technical purposes. 

The works of filling in the details, topographic surveying 
in the closer sense, may be accomplished by various methods, 
differing in the matter of cost, time, and attainable accuracy. 
■ One may be employed with advantage for one class of work, 
while another may be preferable for another class, another locality, 
or, to meet different conditions. The method best adapted to 
any particular region should be employed to obtain the best 
results. 



2 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS, 

The more important methods with their instrumental out- » 
fits are: 

First — ^The direct plotting to scale, in the field, of all fea- 
tures that are to be shown on the chart, 

A — with a plane table and telemeter or stadia rods; 

B — with a tachygraphometer and' stadia rods; 

C — with either of these instruments, but with a leveling 
instrument in addition, for locating the horizontal 
contours ; 

D — using an aneroid barometer in place of the level. 
Second — ^The compilation of all available data (cadastral 
surveys, public land, and count)' surveys, railroad 
and canal surveys, etc.), gi\ing principally the hori- 
zontal distances, a supplementary survey being made 
to furnish the missing data, which in this case are 
principally elevations. They may be supplied by 
trigonometric or spirit leveling, by interpolation and 
sketching. 
Third^-The records of the survey may be obtained in the 
shape of field notes and sketches (" tachymetry "), 
the map being produced by plotting the recorded 
data in the ofl5ce. 

A — with a surveyor's compass and steel tape the rela- 
tive positions of characteristic points may be located 
in the horizontal sense, while their relative eleva- 
tions may be obtained by means of a level, minor 
details being largely sketched; 

B — with a transit and steel tape points are located, both 
geographically and h}'psometrically, minor details 
are sketched; 

C — with a transit and stadia rods; 

D — with a tachymeter and stadia rods, elevations being 
obtained automatically; 

E — adding a leveling instrument for locating horizontal 
contours ; 



mXRODUCTION. 3 

F — using a specially constructed aneroid barometer 
(" Goldschmidt's "), in place of the level for locating 
and tracing the contours in the field. 
Fourth — ^The field records for developing the terrene are 
represented by photographic negatives, taken under 
special conditions from stations of known positions 
and elevations, 
A — with a camera or phototheodolite, using telemeters 
or other distcince measures for obtaining the lengths 
of base lines and a barometer for ascertaining the 
elevations of tertiary points; 
B — with a surveying camera, separate transit, telemeters, 

and barometer; 
C — with a photographic plane table and distance-meas- 
uring alidade, using a barometer for obtaining the 
elevations of detached camera stations; 
D — ^with a surveying camera, separate plane table, dis- 
tance measure, and aneroid barometer; 
E — with a specially constructed phototheodolite, the 
iconometric plotting being done with the Zeiss stereo- 
comparator; 
F — with cameras designed to be used attached to kites, 
to free or to captive balloons. 
Alinute and detailed methods with ensuing accurate results 
should be appHed to the surveys of cities and all densely settled 
regions, to the coastal belts, large river valleys, and lakes, par- 
ticularly when navigable, and this work should be plotted on a 
large scale. 

Arid, barren, and mountainous regions as well as prairies 
and swamp lands, when sparsely settled, should be generalized 
in their cartographic representations and they should be plotted 
on a small scale. 

In exemplification of the preceding suggestions we may 
refer to the new topographic survey of Italy, where Paganini's 
results not only fuUy proved the efficiency of photography applied 



4 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

to surveys of alpine regions (plotted in 1:25000 and 1:50000), 
but also led to the adoption of the phototheodolite as an auxiliary 
instrument to the plane table. The latter was used for mapping 
the areas below 2000 m. in altitude, while the phototheodolite 
was depended on to delineate the terrene lying above that altitude. 

After the area which is to be surveyed has been covered with 
a net of triangles and polygons it will have been provided with a 
framework of lines of known lengths and directions (being in 
itself a skeleton survey of the country), and after the natural 
details and artificial features have been filled in, by one of the 
numerous topographic methods with more or less details and 
accuracy, we will have a topographic survey of the area of more 
or less precision. 

The number of so-called control points for a given area, 
determined in elevation and geographical position during a 
topographic survey, should be increased in the same ratio as 
the degree of accuracy, required for the survey, and also as the 
amount of details, conditioned by the scale of the survey, may be 
increased. 

Photography has been extensively applied to surveys of rugged 
mountain regions in Italy, Austria, Russia, Canada, and Alaska 
with great success. The phototopographic method, originally 
devised by Colonel Laussedat, found its first application in France 
and in Germany. In its early stages it was practiced exclusively 
under governmental and military auspices, being primarily used 
for so-called secret and military surveys. Lately, however, 
phototopography has found a wider and more general applica- 
tion also in France and Germany. 

More recently photographic surveys have been executed in 
Greece, Spain, Portugal, Norway, Belgium, Mexico, Chile, Peru, 
Brazil, Argentine RepubUc, Switzerland, Australia, England, 
Africa, and more recently still in the United States, although 
Lieut. Henry A. Reed has, for several years past, taught photo- 
graphic surveying, theoretically and practically, at the U. S. 
Military Academy at West Point. 



CHAPTER I. 

SHORT REVIEW OF EXECUTED PHOTOGRAPHIC SURVEYS 
AND LISTS OF THE MORE IMPORTANT PUBLICATIONS 
ON PHOTOGRAMMETRY AND PHO TOTOPOGRAPHY. 

I. Photographic Surveying in France. 

Practical results from the early application of photography 
to svirveying failed to materialize for some time, partly owing to 
the slowness and uncertainty of the old photographic process, and 
partly due to the greatly increased efficiency of surveying instru- 
ments and methods in general. 

The chances of utilizing landscape perspectives for a geo- 
metrically true representation of the terrene in horizontal plan 
became realized by the combination of surveying instruments 
with the modem camera (with the reliable, uniform, and quick 
dry-plate process). Of course, it is not necessary to merge the 
camera and the geodetic instnunent into a single apparatus. 
For facilitating transportation and for other reasons they are 
frequently used separately over the same station. 

The theoretical principles upon which photogrammetric 
methods are primarily based were known to J. H. Lambert (of 
Zurich), who published a work on perspective in 1759, ™ which 
reference is made to those identical principles. Still, Lambert's 
suggestions were neither followed nor were his theories given a 
practical application in this respect, until the well-known savant 
and hydrographer Beautemps-Beaupr^, while on a scientific expe- 
dition, in 1 791 to 1793, made a series of free-hand sketches of the 
regions skirting the shores of Van Diemansland (Tasmania) and 
of Santa Cruz Island. ' After his return to France he attempted 

5 



6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the first practical application of Lambert's theory by constructing 
topographic reconnaissance maps of the coastal regions just 
referred to, based upon the outline sketches of the terrene. 

Beautemps-Beaupre subsequently made frequent reference to 
the feasibihty of his cartographic methods, recommending them 
particularly to explorers. Little or nothing, however, was imder- 
taken by others toward improving Beautemps-Beaupre's new 
cartographic method, and it had practically fallen into oblivion 
when Arago, in 1839, called attention to the possibilities of 
photography when utilized in this connection. 

Beautemps-Beaupre's suggestions probably met with so little 
favor because it is not easy to make free-hand sketches of land- 
scapes geometrically accurate enough to be used iconometricaUy in 
place of the landscapes. Iconometry as applied to topographic 
plotting rests upon the same principles as the plane-table method 
of determining points by the intersections of lines of direction, 
drawn to such points from known stations, only in iconometry 
such lines are graphically deduced from the photographic per- 
spectives and are drawn in the office. 

Apparently Capt. Leblanc, of the French Genie Corps, made 
the only appHcations of Beautemps-Beaupre's method, in ante- 
cedence of the year 1849, principally in connection with military 
surveys. 

Col. A. Laussedat had taken up the study of iconometric 
map-plotting in 1850. Li the early part of his investigations 
he utilized a "camera clara" (invented by Wollaston in 1804) 
for obtaining the necessary perspectives of the terrene, tracing 
their outlines by hand. In 1852, however, he replaced that 
instrument by the "camera obscura" (invented by Dom Panunce, 
or by some attributed to G. della Porta). Laussedat's camera 
obscura was modelled after Niepce's, but it was supplied with 
special surveying devices. 

Subsequently as Chef du G^nie Corps, Laussedat executed 
numerous experimental surveys, improving the surveying camera 
and elaborating the methods. In 1858 he obtained a Bertaud 



PHOTOGRAPHIC SURVEYING IN FRANCE. "] 

lens, which was practically free from aberration and which he 
used for the new phototheodolite made by Brunner in Paris. 

In 1859 Laussedat felt justified, by the good results he had 
obtained with this improved phototheodolite, to annoimce the 
successful application of photography to surveying to the Academy 
of Sciences in Paris. After a critical examination of Laussedat's 
methods and results, by Daussy and Laugier, these two members 
of the Academy approved and endorsed his methods, l^aussedat 
also made a few topographic maps with the aid of balloon- 
photography, but soon returned to the exclusive use of the 
station-camera. 

At the exposition of 1867, in Paris, Laussedat exhibited the 
first known phototheodolite and some map specimens, based 
upon photographic surveys, among others a plan of Paris (scale 
1 : 6666) which compared very favorably with one that Emmery 
had made in 1839 by means of instrumental surveys. The 
survey for the phototopographic map of Faverges in Savoy, 
scale 1:5000, was executed in 1867 by Capt. Javary and Lieut. 
Garibaldy, of the Genie Corps, under Col. Laussedat's direction, 
and it was based on 120 photographs. It covered an area of 
about three square miles and the topography was controlled by 
about 5000 points that had been identified on the pictures and 
which were plotted iconometrically on the chart. 

Col. Laussedat's work in this field has been so complete that 
the guiding principles which he first laid down and subsequently 
elaborated by numerous practical applications are still in use, 
and his interest in this work continues unabated to this day. 

From 1 85 1 to 1871 Col. Laussedat and his associates in this 
work were frequently called away from the pursuance of photo- 
topographic surveys, having other duties assigned them, and we 
find that Laussedat's surveying methods did not become generally 
known in France, and it was left to scientists and engineers of 
other countries (Germany and Austria) to popularize this survey- 
ing method and extend its application to various branches of the 
sciences. 



8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

In 1858 Chevallier had an instrument patented under the 
name " planchette photographique " which soon found much 
favor, especially among members of the G6nie Corps. This 
photographic plane table is described by Alophe (" Le pass^, 
le present et I'avenir de la photographic," Paris, 1861), by 
d'Abbadie (" Bulletin de la Soci^te de Geographic de Paris," 
1862), by Pat^ (" Application de la photographic k la topographic 
miUtaire," Paris, 1866), by Col. A. Lausscdat (" Rechcrches 
sur les Instruments, les Methodes et le Dessin topographiques," 
Paris, 1898), and others. It was manufactured by Dubosque, 
in Paris, and used by Wiganowsky, Pate, and A. Jouart. 

Martens, in Paris, was probably the first to devise a " pano- 
ramic camera " (1847), in which he used a cylindrically bent 
daguerreotype plate. 

Inclined plates for phototopography were also used at an early 
date in France, notably by Th. Pujo and T. Fourcade, who 
published their methods, under the title " Goniom^trie Photo- 
graphique," in Les Mondes, No. 4, 1865. 

France had an interesting exhibit at the World's Columbian 
Exposition in Chicago, 1893, showing photographic surveying 
instruments and map specimens, in illustration of topographic 
and astronomical results, gained chiefly under the direction 
of Col. A. Laussedat and taken from the collections of the Con- 
servatoire National des Arts et Metiers, Paris, of which insti- 
tution Col. Laussedat is now director. 

In recent years balloon surveying has been resumed in France 
under the auspices of the Ministry of War, the camera being 
used in connection with both the free and captive balloon. Bal- 
loon surveying had been rather neglected, notwithstanding the 
good results which had been obtained in the early stages of photo- 
graphic surveying in Paris by Col. Laussedat and Nadar (1866). 

Long-distance photography (" telephotography ") seems to 
have been first studied in France by Lacombe and Matthieu. 
A resume of their results has been embodied in an official report 
-to the French Government in 1887 by Commandant Fribourg, 



PHOTOGRAPHIC SURVEYING IN FRANCE. 9 

recommending the adoption of telephotography as a reconnoiter- 
ing method in the Genie Corps. 

Guillemont and Jarret followed in the lines laid down by 
Lacombe and ^latthieu, but little reached the general public 
regarding the practical results obtained by this method in France. 
Still, it is now well known that the French officers stationed 
at Grenoble have obtained excellent results in telephotography 
as applied to mihtary reconnaissance, shoA\'ing the operations and 
the disposition of troops in the field, depicting the effects of 
cannon-shots upon bombarded fortifications, etc. The tele- 
photographic negatives obtained at Grenoble clearly define 
objects at distances from 2 to 6 km. The French telephoto- 
graphic cameras are mostly made by Hondaide and Derogy in 
Paris. 

In 1893 H. VaUot commenced the mapping of the Mont 
Blanc moimtain group and its immediate vicinitj'. He is assisted 
in this work by J. A'allot, who, in 1890, founded a meteorological 
observator}' on ^lont Blanc. This map is being dra'mi in 
1 : 20000 scale and the greater part of the topography is based 
on photographs. 

LITERATURE. 

Other French pubhcations on photography applied to sur- 
veying, besides those already mentioned in the preceding para- 
graphs, are as follows: 

A. Laussedat. "M&noire sur TEmploie de la Chambre Claire dans les 

Reconnaissances topographiques." Memorial de I'Officier du Genie, 

Xo. 16, 1854. 
A. L.ArssEDAT. "Analyse d'un Memoire sur I'Emploie de la Photographie 

dans le Leve des Plans. "Comptes rendus de I'Academie des Sciences, 

1859. 
A. Laussedat. "Memoire sur I'Emploie de la Photographie dans le Leve 

des Plans," Memorial de I'Officier du Genie, Xov. 17, 1864. 
A. Laussed.\t. "Historique de I'Application de la Photographie au 'Lever 

des Plans," "Conference faite au Congres de I'Association Fianjaise 

pour r.\vancement des Sciences," Septr. 17, 1892. Comptes rendus du 

Congres de Pau et Revue scientifique de 1892. 



lO PHOTOTOPOGRAPHIC METHODS AND INSTRDMENTS. 

A. Latjssedat. "L'lconom^trie et la M^trophotographie." ConKrence faite 
au Consen'atoire des Arts et Metiers, deuxieme, Serie, II, III, IV, 1892-93. 

A. Laussedat. "Exposition Universelle de Chicago, 1893, Section Fran- 
Saise, Instruments et Appareils Iconom^triques et M^trophotographiques," 
des Collections du Conservatoire National des Arts et Metiers. Paris, 
1893, Imprimerie Nationale. 

A. Laussedat. "Les Applications de la Perspective au Lever des Plans," 
Annales du Conservatoire des Arts et Metiers. Paris, 1893-94. 

A. Laussedat. "Reconnaissance faite a I'Aide de la Photogiaphie, pour la 
Delimitation de la Frontifere entre 1' Alaska et la Colombie Britanique." 
Comptes rendus de I'Academie des Sciences, Paris, 1894. 

A. Laussedat. " Conference de Metrophotographie," Instructions et Ex- 
plications sommaires a I'Emploie de la Photographic dans les Recon- 
naissances topographiques faites par les Voyageurs," Revue scientifique. 
No. 26 I, No. 23 II. Paris, 1894. 

A. Laussedat. "Recherches sur les Instruments, les Methodes et le Dessin 
topographiques" (Tome II). Paris, 1903. 

A. Laussedat. "De I'Emploi du Stereoscope en Topographic et en Astro- 
nomic." Comptes rendus des Seances de I'Academie des Sciences, 
CXXXVI, p. 22-28, 1903. 

A. Laussedat. "Sur un Moyen rapide d'obtenir le Plan d'un Terrain 
en Pays de plaines, d'aprfes une Vue photographique prise en Ballon." 
Comptes rendus, 137 Vol., pp. 24-30. Paris, 1903. 

E. Pate. "Applications de la Photographic k la Topographie miUtaire." 
Paris, 1862. 

Laugier &Daussy. "Rapport sur IcMemoiredcM. Laussedat.'' Comptes 
rendus de I'Academie des Sciences, i860. 

A. Jouart. "Applications de la Photographic au Lever militaire." Paris, 
1866. 

Javaey. "Le Memoire sur les Applications de la Photographic aux Arts 
militaires." Memorial de I'Officicr du Genie. Paris, No. 22, 1874. 

Dr. G. Le Bon. "Les Civilizations de I'Indc." Finnin, Didot et Cie. 
Paris, 1887. 

Dr. G. Le Bon. "Les Levers photographiques et la Photographic en 
Voyage.'' Gautier Villars et Fils. Paris, 1889. 
Partie I: "Application de la Photographic a I'^tude geomctrique des 

monuments et a la Topographie." 
Partie II: "Operations complementaires des Levers topographiques." 

P. MoESSARD. "Le CyUndrographe, Appareil panoramique." 

Partie II: "Lc CyUndrographe topographique." Gauthicr, Villars et 
Fils. Paris, 1889. 

P. MofssARD. "Les Panoramas photographiques ct les Appareils pano- 



PHOTOGRAPHIC SURVEYING IN FRANCE. II 

ramiques." Bulletin de la Socidtd Franjaise de Photographic, deuxifeme 

Sdrie, Tome IX, 1893. 
E. Monet. "Principes fondamentaux de la Photogramm^trie." Soci^t^ 

d'Editions scientifiques. Paris, 1893. 
E. Monet. "Application de la Photographic ^ la Topographic." Bulletin 

d'Aodt, 1894, de la Soci^t^ des Ing^nieurs Civils de France. 
J. GiRARD. "La Photographic et ses Applications aux Etudes g^ographiques." 

Paris, 1872. 
Alfred Hanoi. "La Photographic dans les Arm&s." Paris, 1875. 
Fl. Dumas. "De la Photographic et ses Applications aux Besoins de 

TArmfe.'' Paris, 1872. 
J. BoRNECQUE. "La Photographic appliqufe au Lever des Plans.'' Paris, 1886. 
A. DE LA FuYE. "La Photographic k grandc Distance." Revue du Ccrcle 

militaire, 1895. 
A. DE LA FuYE. "M^moire sur I'Emploi des Appareils photographiques 

pour les Observations k grandc et k petite Distances." Autographic S. 

r^^cole de GCnie dc Grenoble, 1891. 
Capt. E. Deville. "Lever topographique des Montagnes Rocheuses, exe- 
cute par la Photographic." Bulletin dc la SocidtC franjaisc de Photo- 
graphic, deuxifeme SCrie, 1893. 
R. Colson. "La Photographic sans Objective." Gauthier, Villars et Fils. 

Paris, 1887. 
Arthur Batut. "La Photographic aCrienne par Cerf volant." Gauthier, 

Villars et Fils. Paris, 1890. 
Lc Col. Berthaud, Chef dc la Section topographique. "La Carte de 

"France (1750-1898). feude historique," 2 Vol., Imprimerie du Service 

gfographique de PArmfe, 1899. 
Le Capt. du G6nie Bouttr^aux. "MCmoire sur la TCldphotographic," 

Revue du Gdnie, Sept., 1897. 
L. Cazes. "St&6oscopie de Precision. Thforie et Pratique," Librairie 

Michelet, Ph. Pellier, lEditeur, 1895. 

E. Douglas-Archibald. "Les Cerfs volants militaires." Librairie Uni- 

verscUc. Paris, 1888. 

F. Drouin. "Le St&doscope et la Photographic st&Coscopique." Ch. 

Mendel. Paris, 1894. 

Le Capt. Houdaille. "Sur une MCthode d'Essai scientifique et pratique 
des Objectifs photographiques " Gauthier, Villars et Fils. Paris, 1894. 

J. Le Cornu. "Les Cerfs volants." Monie et Cie. Paris, 1902. 

MaxLoehr. "Surla DdterminationdesM&uresduTClCobjcctif." Bulle- 
tin de la Society franjaise de Photographic, 1902. 

H. RoussoN. "Instructions sur le Photogrammftre de I'Explorateur." 
Imprimerie Dubos. Paris, 1900. 



12 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Gaston Tissaistdier. "La Photographic en Ballon " Gauthier, Villars et 

Fils. Paris, 1886. 
A. Vautier-Dtifoue. " Sur la Tdl^photographie." Bull, de la See. Vaudoise 

des Sciences naturelles, No. 143. Corbaz et Cie. Lausanne, 1902 
Ch. von Zeegler. "Le Perspecteur m^chanique." Fr. Weber. G^nfeve 

igoo. 
E. Wallon. "Traits eltoentaire de I'Objectif photographique." Gauthier, 

Villars et Fils. Paris, 1891. 
E. Wallon. "Choix et Usage des Objectifs photographiques." Gauthier, 

Villars et Fils. Paris. 
Emle Wenz. ' ' Resume historique de I'lnvention de la Photographie aii ienne 

par Cerfs volants." Bull, de la Soc. franfaise de Photographie. Avril, 

1902. 



II. Photographic Surveying in Germany. 

Col. Laussedat's photographic surveying methods soon found 
admirers and earnest students in Germany and Dr. A. Meyden- 
baur became an early exponent of this method. Some writers 
even claim that, in 1858, while Meydenbaur was engaged with 
obtaining the measurements that were" needed for planning 
the renovation of the cathedral at Wetzlar, he had, independ- 
ently of Col. Laussedat's work, conceived the idea to utilize 
photographs of the building to arrive at the dimensions of inac- 
cessible details. 

General von Alster repeatedly recommended in his official 
reports that experimental surveys be inaugurated by the Prussian 
Government, to familiarize the officers of the General Staff with 
the photographic surveying methods, but the war of 1866 pre- 
vented action being taken in this direction. Still, Count von 
Moltke, Chief of the Prussian General Staff, had soon recog- 
nized the value of such methods for military and secret surveys^ 
and before the next war took place (which was the Franco- 
Prussian war of 1870-71), a complete military phototopographic 
detachment had been formed and organized. In 1870 this 
reconnoitering corps was placed under the command of Capt. 
Bemhardi and Lieut. R. Doergens, who furthermore had the 



PHOTOGRAPHIC SURVEYING IN GERMANY. 1 3 

assistance of the ci\'il photographic experts Hintze, Quidde, 
and Schmier. 

During the siege of Strassburg this corps made a survey 
of a part of the city and the near-by fortifications, plotting the 
work in 1:2500 scale, but the early surrender of the city made 
it imnecessary to finish the survey. Only a few ranges as obtained 
by this survey had been utilized by the army, principally to 
concentrate the bombardment to certain districts, sparing others, 
especially the noble cathedral building. During the icono- 
metric plotting of this map some discrepancies were discovered, 
which Dr. Doergens ascribed to defects in the camera lens and 
to the unstable character of the instrument, which had been 
put together too hurriedly. 

No further opportunities were offered during the Franco- 
Prussian war for other phototopographic surveys and the sur- 
veying cameras were subsequently used for obtaining pictures 
of historically interesting features, of points of strategic impor- 
tance to be utilized in making corrections and additions to the 
maps of France which had been distributed among the ofl&cers 
that were directing the movements of the invading German armies. 

Probably the first German pubHcation on the subject of 
photographic surveying maybe found in Horn's " Photographische 
MittheUimgen," April, 1863, being a German translation of Col. 
Laussedat's explanations given in his lecture, dehvered Jan. 9 
1863, before the "Societe photographique " in Paris. 

In the June number of the same magazine Dr. Meydenbaur 
published his first article on photographic surveying, in which 
he makes use of the term "photometrographie". Subsequently 
this was changed to "metrophotographie" and finally to "photo- 
grammetrie" ("Bildmesskunst"). 

The March number of Horn's "Photographische Mitthei- 
lungen," 1866, contains an article by Dr. Vogel on Johnson's 
photographic apparatus for making topographic surveys, in which 
the method for the iconometric determination of horizontal and 
vertical angles is described. 



14 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Since 1882 Dr. Meydenbaur has been the Director of the 
" Photogrammetric Institute" in Berlin, founded by the Prussian 
Government for the collection of photographic data to bring the 
German topographic maps up to date and fOT the preservation 
of views of ancient monuments and buildings. Dr. Meydenbaur 
is now engaged with the publication of " Das Denkmaler Archiv," 
being a collection of photographic reproducnons of buildings of 
archeological and architectural interest. 

Since 1867 a number of photographic surveys have been made 
under the auspices of the Prussian Ministry of War. The first 
larger attempt was the topographic survey of Freiburg and vicinity, 
scale 1:1000, including the architectural survey of the cathedral 
of the same city. 

From 1869 the theory of photographic surveying has been 
included in the lectures on geodesy (Prof. Doergens) at the Royal 
Building Academy in Berlin. In 1886, when the new Royal 
Technical High School at Charlottenburg, near Berlin, had been 
completed, a special chair (filled by Dr. Pietsch) was set aside 
for teaching photogrammetry in all its branches, including bal- 
loon surveying. 

The late Prof. Jordan, as a member of G. Rholf's African, 
exploring expedition in 1873-74, made a phototopographic sur- 
vey of the "Oasis Dachel, " including the settlement "Gassr- 
Dachel," in the Libyan Desert. (Jordan, "Vermessungskimde," 
Vol. II, Stuttgart, 1893, and ." Zeitschrif t fuer Vermessungs- 
wesen," Part I, Vol. V, 1876.) 

Prof. Jordan fuUy appreciated the value of photography as an 
auxiliary to topographic surveys, and he remarks in the article 
"Ueber die Verwerthung der Photographie zu geometrischen 
Aufnahmen" (Zeitschr. f. Verm., 1876): . . . "Dasz die Photo- 
graphie in vielen gewissen Faellen mit ausserordentlichem Vor- 
theil angewandt werden koennte, z. B. bei schwer zugaenglichen 
Gebirgen und auf Entdeckungsreisen, erscheint beim ersten 
Blick auf die Sache zweifellos." 

The good results obtained by Dr. Meydenbaur in the valleys- 



PHOTOGRAPHIC SURVEYING IN AUSTRIA. IJ 

of the Reusz (1873) ^^^ Rhine (1876) considerably increased the 
number of advocates for phototopography in Germany. 

Dr. Stolze in 1874, while a member of the archaeological' 
expedition under F. C. Andreas, used a Meydenbaur camera- 
theodolite to make a survey of the ruins of Persepolis, of Pasarga- 
dae, and of the ancient temple at Djamhat ("Masdjid i Djam^ht")' 
in Shiraz, Persia. 

Dr. S. Finsterwalder has made several phototopographic sur- 
veys in the Bavarian Alps, including surveys of glaciers, made at 
stated time-intervals, to ascertain their variations and movements 
in both the horizontal and the vertical sense. During the summer 
months of 1888 and 1889 he surveyed the "Vemacht" Glacier in 
the valley of the Oetz, in Tyrol, having the assistance of Dr. A. 
Bluemke and Dr. H. Hess; in later surveys he was assisted by 
Dr. Kerschensteiner. 

Dr. C. Koppe has done much literary and practical work in 
photogrammetry and phototopography. Recently he has pub- 
lished the description of his new phototheodolite, made by F. 
Randhagen in Hannover, that may also be used for astronomical 
observations (lat. determinations). 

Dr. Meydenbaur, Prof. Jordan, Dr. Doergens, Dr. Stolze, 
Dr. Schroeder, Dr. Vogel, Dr. Koppe, Dr. Hauck, Prof. Finster- 
walder, Prof. Foerster, Dr. Pietsch, Dr. Voigtlaender, and others 
have contributed largely toward the popularization of the photo- 
topographic methods in Germany. 

in. Photographic Surveying in Austria. 

. Karl Koristka, having made the acquaintance of Messrs. 
Laussedat and Chevallier in Paris in 1867, became interested in 
photography applied to surveying, and he probably is the first 
who made a phototopographic survey in Austria (survey of the 
city of Prague). Still, the method met with little favor in Austria 
owing to the inconvenience of the wet-plate process, until, in 1890, 
the authorities of the Military Geographic Institute, in Vienna, 



36 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

ordered a series of experimental photographic surveys to be made 
in the vicinity of Vienna, which fully demonstrated the usefulness of 
this method for the surveys of certain regions. Among the offi- 
cers of the Austrian army we may mention Lieut. L. Mikiewicz, 
Maj. Bock, Maj. PizzigheUi, Lieut. Hartl, Capt. Huebl, and others 
who have materially aided in the development of photographic 
surveying methods in Austria. 

Since 1889 many engineers have adopted phototopographic 
methods for the surveys of inaccessible regions, or for regions 
where the other topographic methods would have been too time- 
consuming or too expensive. Among these we may cite V. 
Pollack (Eng'r in Chief of the Austrian Government R.R. System), 
M. Maurer, F. Haflerl, etc. 

Prof. F. Schiffner, Prof. A. Schell, and Prof. F. Steiner (the 
latter teaches the principles of photographic surveying at the Tech. 
High School in Prague) have done much as teachers, writers, and 
propagators in improving phototopographic methods and instru- 
ments. 

LITERATURE (German and Austrian). 

Archiv fuer die Offiziere des Kgl. Preuss. Artillerie u. Ing. Corps, 1868. 

Deutsche Bauzeitung, 1872. 
J. Girard. "Laussedat's Arbeiten in Bezug auf die Anwendung der Photo- 

graphie zur Aufnahme von Plaenen." Photographisches Archiv, Sept., 

1865. 
Dr. A. Meydenbaur. "Ueber die Anwendung der Photographie zu Archi- 

tectur und Terrain Aufnahmen." Erbkam's Zeitschr. f. Bauw., 1867. 
Dr. A. Meydenbaur. "Das Photographische Aufnehmen zu wissenschaft- 

lichen Zwecken, ins Besondere das Messbildverfahren." Unte's Verl. 

Berlin, 1892. 
Dr. A. Meydenbaur. "Zum gegenwaertigen Stande des Messbildver- 

fahrens." Deutsche Bauz., 1894. 
Dr. A. Meydenbaxjr. "Ueber Behandlung grosser Flatten auf Reisen." 

Photographisches Wochenblatt, 1888. 
T>r. PiETSCH. "Die Photogrammetrie." Zeitschr. f. Verm., Heft 23 and 24 

1887. ' 

Prof. W. Jordan. "Ueber die Verwerthung der Photographie zu geome- 

tiischen Aufnahmen." Zeitschr. f. Verm., Heft I, 1876. 



PHOTOGRAPHIC SURVEYING IN AUSTRIA. 1 7 

Prof. Stolze. "Photographisches Wochenblatt," 1881, 1882, and 1885. 

Prof. Stolze. "Photogrammetrie." "Das Licht im Dienst wissenschaft- 
licher Forschung," Dr. S. T. Stein, Bd. II, 1888. V>m. Knapp. 

Dr. H. W. VoGEL and Prof. Dr. G. Doergens. "Ueber einen einfachea 
photogrammetrischen Apparat." Photogr. Mitt., 1884. 

Dr. H. W. VoGEL. "Astronomische Nachrichten," 1888. 

Dr. R. Doergens. "Zur Pruefung und Berichtigung des photogramme- 
trischen Apparates." Phot. :Mitth., 1886. 

Dr. R. Doergens. "Ermittelung der Constanten des photogrammetrischen 
Apparates." Phot. Mitth., 1886. 

Dr. R. Doergens. "Ueber Photogrammetrie und Ueber die Thaetigkeit des 
Feld-Photographie Detachments im Kriege 1870-71." K.' Schmier. 
Weimar, 1897. 

Dr. FiNSTERWALDER. "Die Terrainaufnahmen mittels Photogrammetrie." 
Bayersches Industrie und Gewerbebl. Muenchen, 1890. 

L. MiKiEWicz. "Anwendung der Photographie zu militairischen Zwecken." 
Mitth. ueber Gegenst. d. Artillerie u. Geniewesens, 1876. 

G. Pizzighelli. "Handbuch d. Photographie," Bd. II, 1887. Knapp- 
Halle, a./S. 

G. Pizzighelli. "Die Photogrammetrie." Mitth. ueber Gegenst. d- 
Artillerie u. Geniewesens, 1884. 

Dr. C. KOPPE. "Die Photogrammetrie oder Bildmesskuast." Weimar, 
1889. 

Dr. C. KoppE. "Photogrammetrie und Internationale Wolkenmessung." 
Braunschweig, 1896. 

Dr. G. Hauck. "Neue Constructionen der Perspective und Photogram- 
metrie." Journal fuer reine und angewandte Mathematik, 1883. 

Dr. G. Hauck. "Theorie der trilinearen Verwandtschaft ebener Systeme." 
Journal fuer reine und angewandte Mathematik, 1883. 

Dr. G. Hauck. "Mein perspectivischer Apparat." Festschrift der Koenigl.. 
techn. Hochschule zu Berlin, Zur Feier der Einweihung ihres neuen 
Gebaudes, am 2ten Nov., 1884, Reichsdruckerei. Berlin, 1884. 

Herm. Ritter. "Perspectograph." Apparat zur mechanischen Herstellung 
der Perspective, etc. Maubach u. Co. Frankfurt a./M. 

Prof. J. Heller. "Ueber Photogrammetrie." Ber. d. Ver. d. Techniker 
in Ober-Oesterreich. Linz, 1890. 

Prof. J. Heller. "Ueber neue Erscheinungen auf dem Geziete der Photo- 
grammetrie." Ber. des Ver. d. Techniker in Oberoesterreich. Linz,_ 
1891. 

Dr. ScHROEDER. "ArchtectuT u. Gelandeaufnahme unter Mitwirkung der 
Photographie und der einschlagigen Instnimente." Archiv fuer .A.rtil- 
lerie- und Genie-Ofiiziere des deutschen Heeres. JuH Heft, 1892. 



1 8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Dr. ScHROEDER. "Die neusten Messbild-Instrumente." Arcihv f. Artil- 
lerie- und Genie-Ofilziere des deutschen Heeres. October u. November 
Hefte, 1892. 

V. Pollack. "Ueber Anwendung der Photogrammetrie im Hochgebirge." 
Wochenschrift des Oesterreichischen Ing. u. Archit. Vereins, 1890. 

V. Pollack. "Die photograph. Terrain-Aufnahmen mit Beruecksichtigung 
d. Arbeit in Steiermark." R. Lechner. Wien, 1891. 

v. Pollack. "Photogrammetrie oder Phototopographie." Mitth. d. kgL 
kaiserl. Geogr. Gesellschaft. Wien, 1891. 

V. Pollack. "Ueber Photogrammetrie." Zeitschr. d. Ver. deutscher In- 
genieure, 1893. 

v. Pollack. "Ueber photogr. Messkunst, Photogrammetrie und Photo- 
topographie." Mitth. d. kgl. kaiserl. Geographischen Gesellschaft. 
Wien, 1891. 

V. Pollack. "Ueber Photogrammetrie und deren Entwickelung." Monats- 
blaetter des wissenschaftlichen Clubs in Wien, No. 5, Bd. XIII, 
1891. 

v. Pollack. "Die photographischen Terrainaufnahmen." Centralblatt 
fuer das gesammte Forstwesen. Wien, 1891. 

v. Pollack. "Ueber Fortschritte der Photogrammetrie.'' Dr. Eder's 
Jahrb. fuer Photographie. Wien, 1892. 

V. Pollack. "Die Beziehungen der Photogrammetrie zu den topograph. 
Neu-Aufnahmen im Bairisch-oesterreichischen Grenzgebirge." Archiv 
f. Artillerie- u. Genie-Offiziere des deutschen Reichsheeres, 1893. 

V. Pollack. "Photograimnetrische Arbeiten in Oesterreich." Dr. Eder's 
Jahrbuch fuer Photographie, 1894. 

V. Pollack. . "Ein neuer durchschlagbarer Phototheodolit mit centrischem 
Fernrohr." Zeitschr. d. oester. Ing. u. Archit. Vereins, 1894. 

F. Steiner. "Ueber Photogrammetrie und deren Anwendung.'' Tech- 
nische Blaetter. Prag, 1890 and 1891. 

F. Steiner. "Das Problem der fuenf Punkte, eine Aufgabe der Photo- 
grammetrie." Wochenschr. d. oester. Ing. u. Archit. Vereins, 1891. 

F. Steiner. "Die Photographie im Dienste des Ingenieurs." Ein Lehrbuch 
der Photogrammetrie. R. Lechner. Wien, 1891-94. 

F. Steiner. "Die Anwendung der Photographie auf dem Gebiete des Bau- 
und Ingenieurwesens, mit besonderer Beruecksichtigung der Photogram- 
metrie." Technische Blaetter, Heft III u. IV. Prag, 1891. 

M. Bock. "Die Photogrammetrie." Mitth. ueber Gegenst. d. Artillerie 
und Genie Heft I. Wien, 1891. 

R. KoBSA. "Die Photogrammetrie oder Bildmesskunst, und speciell deren 
Verwendung im Dienste des Forsteinrichters." Oesterreich. Viertel- 
jahrschrift fuer Forstwesen, Heft II, 1892. 



PHOTOGRAPHIC SURVEYING IN AUSTRIA. 1 9 

F. Hafferl. "Das Teleobjectiv und seine Verwendbarkeit zu photogram- 
metrischen Aufnahmen." Zeitschr. fuer Verm., 2istes Heft, 1892. 

F. Wang. "Die Photogrammetrie im Dienste des Forsttechnikers." Mitth. 
d. krainischen kuestenlaendischen Forstveriens. Laibach, 1893. 

F. Wang. " Photogrammetrische Instrumente." Oesterreichische Forst- 
zeitung, Heftc i, 2, 3, 1893. 

F. Wang. "Die Anwendung der Photogrammetrie im forstlichen Haus- 

halte." Oesterreichische Forstzeitung, Hefte 19, 20, und 21, 1892. 

E. DoLEZAL. "Die Anwendung der Photographic in der practischen Mess- 

kunst." W. Knapp. Halle a./S., 1896. 
Dr. J. W. Eder und E. Valenta. "Photogrammetrie." Dinglers Polytech. 

Journal, 1892. 
Dr. J. W. Eder. "Photogrammetrische Apparate und Phototheodolite." 

Lehrbuch der Photographie, I. Bd., 1892. 
O. J. Klotz. "Photogrammetrische Arbeiten in Canada." Zeitschr. des 

oesterreich. Ing. u. Archit. Vereins. Wien, 1894. 

G. Starke. "Phototheodolit von Starke und Kammerer." Zeitschr. d. 
oesterreich. Ing. und Archit. Vereins. Wien, 1894. 

Von Huebl. "Messbild Photogrammeter." Annalen fuer Gewerbe und 
Bauwesen, 1892. 

Von Htjebl. "Messtisch Photogrammeter." Lechner's Mitth. aus dem 
Gebiete der Photographie und Kartographie." Graben 31. Wien. 

Imfeld. "Ueber Photogrammetrie." Schweizer Bauzeitung, 1893. 

Dr. A. MiETHE. "Photographische Mittheilungen." Jahrgang 24. 

VoLKMER. "Das Wesen der Photogrammetrie." Wochenschr. d. oester- 
reich. Ing. u. Archit. Vereins, Bd. XIV. 

G. Fritsch. "Anieitung zu wissensch. Beob. auf Reisen." Dr. G. Neu- 
mayer. Berlin, 1888, R. Oppenheimer. 

F. ScHiFFNER. "Ueber Photogrammetrie und ihre Anwendung bei Terrain- 

aufnahmen." Mitth. aus dem Gebiete des Seewesens; 1887. 
F. ScHiFFNER. "Ueber photogrammetrische Aufnahmen mit gewoehnlichen 

Apparaten." Photographische Correspondenz, 1889. 
F. ScHiFFNER. "Ueber photographische Messkunst." Organ der militair- 

wissenschaftlichen Vereme, 1888 und 1889. 
F. SCHIFFNER. "Ueber die photogrammetrische Aufnahme einer Kueste 

im Vorbeifahren." Mitth. aus dem Gebiete des Seewesens, 1890. 
F. SCHIFFNER. "Die photographische Messkunst, Photogrammetrie, Bild- 

messkunst und Phototopographie." W. Knapp. Halle a./S., 1892. 
F ScHiFFNER. "Photogrammetrische Studien." Photographische Rund- 
schau, 1890 und 1891. 
F. SCHIFFNER. 'Forcschritte der Photogrammetrie." Dr. Eder's Jahrb. 

fuer Photographie, 1891. 



20 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

F. SCHTFFNER. "Die Fortschritte der photographischen Messkunst im 

Jahre 1889." Photogr. Rundschau, 1890. 
A. R. VON GuTTENBERG. "Die Photogrammetrie im Dienste der Forst- 

messung." Oesterreich. Forstzeitung, April 17, 1891. 
"Photographische Rundschau," Heft 7, 1891; Heft 6, 9, 11, 1892. Wien. 
C. Tronquoy. "Bemerkungen ueber die [planchelte photographique] von 

Chevallier." Photographische Correspondenz, 1867. 
"Photographische Correspondenz," No. 353, 1890; Seite 380-384, 1892. 
Von Htiebl. "Die Stereophotogrammetrie." Mitth. d. k. u. k. militair- 

geogr. Instituts, XII, 1903. 
A. SCHWASSMANN. "Der Stereokomparator." Annalen der Hydrographie 

u. maritimen Meteorologie, Juli, 1902. 
Prof. Becker. "Ueber Relieffemrohre u. Entfemungsmesser von C. Zeiss." 

Schweizer Zeitschrift fuer Artillerie u. Genie, Seite 365, 1900. 
E. Hering. "Ueber die Grenzen der Sehschaerfe." Berichte der mathem. 

phys. Classe der koenigl. Sachs. Gessellsch. der Wissensch. Leipzig, 

1899. 
L. HEiiirE. "Sehschaerfe und Tiefenwahrnehmung." Von Graefe's Archiv 

fuer Ophthaknologie, No. 51, 1900. 
Dr. O. Hecker. "Ueber die Beurtheilung der Raumtiefe und den stereo- 

skopischen Entfemungsmesser von C. Zeiss." Jena, Zeitschr. fuer 

Verm., No. 30, 1901. 
Von Huebl. "Die stereophotogrammetrische Terrainaufnahme." Mitth. 

des k. und k. militair-geographischen Institutes. Wien, 1903. 
W. Laska. "Ueber eine neue Phototheodolitkonstruction." Zeitschr. fuer 

Instrumentenkunde, 1903. 
W. Laska. "Ueber ein Problem der photogrammetrischen Kuestenauf- 

nahme." Monatshefte fuer Mathem. und Physik, Bd. 12. Wien; 
A. Sprung. "Ueber die allgemeinen Formeln der Photogrammetrie." 

Meteorologische Zeitschrift, 1903. 
R. Grimsinski. "Aufnahme von Horizontalkurven durch das Nivellier- 

Messtischverfahren." AUgemeine Vermessungsnachrichten, Seite 252 

und 253, 1903. 
S. Finsterwalder. "Neue Methode zur topographischen Vermessung von 

Ballonaufnahmen." Sitzungsberichte der Muenchener Akademie, 1903. 
Dr. C. Ptjlprich. "Ueber einen Versuch zur praktischen Erprobung der 

Stereophotogrammetrie fuer die Zwecke der Topographic." Zeitschrift 

fuer Instrumentenkunde, Seite 317-334, 1903. 
Dr. C. PuLFRiCH. "Ueber neuere Anwendungen der Stereoskopie und ueber 

einen hierfuer bestimmten Stereokomparator." Zeitschr. fuer Instru- 

mentenk., XXII, igo2, Seite 65-81; 133-141; 178-192; 229-246. 
Dr. C. PuLFRiCH. "Ueber die Constructionen von Hoehenkurven und Planen 



PHOTOGRAPHIC SURVEYING IN SWEDEN. 21 

auf Grund stereophotogrammetrischser Messungen mit Hilfe des Stereo- 
comparators." Zeitschrift fuer Instrumentenkunde, XXIII, 1903. 
Dr. C. PuLFRiCH. "Neure stereoskopische Methoden und Apparate fuer die 

Zwecke der Astronomie.'' Topographie und Metronomie. Berlin, 1903. 
Dr. C. PuLFRiCH. "Ueber eine neue Art der Herstellung topographischer 

Karten und ueber einen haerfuer bestimmten Stereoplanigraphen." 

Zeitschr. fuer Instrumentenkunde, XXIII, 1903. 
Dr. C. PuLFRicH. "Herstellung telestereoskopischer Landschaftsaufnahmen 

mit Hilfe einer gewoehnlichen Stereokammer." Photographisches 

Centralblatt, Heft 24, VIII, 1902. 
Dr. C. PuLFRicH. "Ueber eine Pruefungstafel fuer stereoskopisches Sehen." 

Zeitschrift fuer Instrumentenkunde, Heft 9, 1901. 
Dr. C. PuLFRiCH. " Ueber einige stereoskopische Versuche.'' Zeitschrift fuer 

Instrumentenkunde, Heft 21, 1901. 
Julius Mandl. "Ueber die Verwerthung von photographischen Aufnahmen 
. aus dem Luftballon." Mitth. ueber Gegenst. des ArtUlerie- imd Genie- 

Wesens, XXIX Jahrgang. Wien, 1898. 
O. Jesse. "Untersuchungen ueber die sogenannten leuchtenden Wolken." 

Sitzungsberichte der koenigl. Preuss. Akademie d. Wissensch. zu Berlin. 

1890-91. 
Dr. K. Heun. "Die Bestimmung der Geschwindigkeit nach den Methoden 

der Photogranmietrie." Schlomilchs Zeitschrift fuer Mathem. u. Phys., 

Bd. XLIV, 1899. 



rV. Photographic Surveying in Sweden. 

Swedish meteorologists made use of photographs for the 
study of cloud formations in 1877, and since then H. Hildebrand 
Hildebrandson, Director of the Meteorological Observatory 
at Upsala, has used and recommends the photogrammetric 
methods for observations to ascertain cloud altitudes and air- 
currents. Identical points of the same cloud, photographed 
on two simultaneously exposed plates, may be found at leisure 
and with a far greater degree of certainty than such points may 
be located, in nature, by two observers with transits and tele- 
phone connection for a mutual guidance toward the selection 
of identical points for observation. 

The first attempt to apply phototopographic methods in 



22 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Sweden was probably made by Prof. G. de Geer in 1882, when 
he, together with Prof. A. G. Nathorst, surveyed some glacial 
fields in Spitzbergen. 

Since 1890 the Swedish General Staff has been actively engaged 
with phototopographic surveys, notably under the direction of 
Major Lowison. The field work is conducted very much in 
the same manner as that executed in Italy under L. P. Paganini, 
restricting the use of the camera to the mountain regions above 
timber-line, the valleys and wooded areas being surveyed with 
the plane table. 

Among those who have actively used phototopographic meth- 
ods in Sweden we may cite Major H. Kinberg, Prof. Rosen, 
Dr. A. Hamberg, Major N. C. Ringertz, J. Westmaim, and 
others. 

LITERATURE. 

H. H. HiLDEBRANDSON. "SuT la Classification des Nuages employfe k 

rObservatoire m^tdorologique d'Upsala.'' Photographies de Henri Osti. 

Upsala, 1879. 
Nils EcKHOLMet H. L. Hagstrom. "Mesures des Hauteurs et des Mouve- 

ments des Nuages." Rapport au Comite mfteorologique international. 

Lisbonne et Upsala, 1885. 
H. H. Hn-DEBBANDSON et H. L. Hagstrom. "Des principalis Mdthodes 

employees pour observer et Mesvu-er les Nuages." Upsala, 1893. 
Nils Eckholm. "Einige Bemerkungen ueber die Anwendung der Photo- 

grammeter zur Messung von Wolkenhohen." Meteorolog. Zeitschrift, 

1894, Seite 377. 
Nils Eckholm. "A new Instrument for Cloud-measurements." Quarterly 

Journal of the Royal Meteorological Society, XIX, 1893. 
Ph. Ackeeblom. "De I'Emploi des Photogrammfetres pour mesurer la 

Hauteur des Nuages." Upsala, 1894. 
K. P. Olsson. "On the Calculation of Photographic Cloud-measurements." 

Quarterly Journal of the Royal Meteorological Soc, Vol. XX, 1894. 
E. LuNDAL et J. Westmann. "Mesures photogrammfetriques des Nuages h. 

Upsala, 1896-97. Upsala. 
Prof. E. Dolezal. "Photogrammetrische ArbeiteninSchweden." Zeitschr. 

fuer Verm., Bd. XXXII, Heft 10, 1903. 
KoNiGL. Krigsvetenskaps-Akademiens Handlihgar och Tidskrift, Nos. 11 

and 12. Stockholm, 1901. 



PHOTOGRAPHIC SURVEYING IN SWITZERLAND. 23 

P. AcKERBLOM. "Ueber die Anwendung der Photogrammetrie zur Messung 
von Wolkenhoehen." Meteorologische Zeitschr., 1894. 



V. Photographic Surveying in Switzerland. 

Since 1889 Prof- Becker, Col. Fahrlander, Prof. Amrein, 
and others suggested the use of photographic methods for bring- 
ing the general topographic maps of Switzerland up to a standard 
to meet modern requirements. The Topographic Bureau finally 
entered upon a series of experimental surveys to ascertain whether 
the phototopographic methods had reached that stage of per- 
fection that they should replace the plane-table methods here- 
tofore in use for alpine topography. 

A description of the phototopographic experimental sur- 
veys, made imder the auspices of the Topographic Bureau of 
Switzerland, may be found in: 

M. RosENMtTND. "Uiitersuchungen ueber die Anwendung des photogram- 
metrischen Verfahrens fuer topographische Aufnahmen." Fritz Haller. 
Bern, 1896. 

The Topographical Engineer, S. Simon, used the photo- 
topographic method extensively in connection with the pre- 
liminary surveys of location for the " Jungfrau " railroad and 
for improving the terrene representation of some of the General 
Staff maps of Switzerland. 

The following publications bearing on this subject may 
be cited: 

S. Simon. "Le Projet de Chemin de Fer de la Jungfrau, examind au point 
de vue scientifique, technique et financier." F. Schultheiss. Zurich, 
1897. 

S. Simon. "Photogrammetrische Studien und deren Verwerthung bei den 
Vorarbeiten fuer die Jungfrau Bahn." Schweizer Bauzeitung, Hefte 
23. 24, u. 25, 1895. 

S. Simon. "Photogrammetrische Arbeiten fuer die Jungfrau Bahn." Schwei- 
zer Bauzeitung, Hefte 11 u. 12, 1896. 



24 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



VI. Photographic Surveying in Italy. 

The extensive mountainous regions of Italy are peculiarly 
well adapted for the application of photography to their topo- 
graphic surveys and we find that phototopography for several 
decades past has been practiced in that country with marked 
success. 

Prof. Porro spent much time, labor, and energy towards 
perfecting the methods and instruments to apply photography 
to tachymetry and to topography. The results of his investi- 
gations, which date back to the year 1853, were published under 
the title " Applicatione della Fotografia aUa Geodesia " (Sal- 
dini, Milano, 1855). 

Prof. Porro's instruments, which were supplied with sen- 
sitized plates of spherical shape (" Fotografia sferica "), have 
been preserved by Salmairaghi, Director of the Polytechnic 
Institute in Milan, Prof. Porro having been a member of the 
faculty of this institution. 

With Porro's death further investigations and experiments 
in phototopography ceased in Italy until the year 1875, when 
Michele Manzi, an oflScer of the Mihtary Geographic Institute 
of Italy, utilized some photographs of views, taken in the Abruzzi 
Mountains with an ordinary camera, to supplement the topo- 
graphic details of his plane-table survey of the Gran Sasso 
group. This attempt gave such gratifying results that the 
same officer in the following year made a special and more prac- 
tical application of photography in the topographic survey 
of Mont Cenis, particularly of the region about the Bart 
Glacier. 

In 1878 Gen. Ferrero, Chief of the Geodetic Department 
of the Military Geographic Institute, pointed out to the Direc- 
tory that the resumption of practical tests and experimental 
surveys in phototopography were very desirable, in view of the 
advances which had been made in the photographic methods 



PHOTOGRAPHIC SURVEYING IN ITALY. 25 

recently. Phototopography had been suspended in deference 
to the claims of several members of the Institute that photo- 
graphic data were too imreliable for topographic purposes, par- 
ticularly for large scale maps. 

In the same year (1878) L. P. Paganini, Engineer Geographer 
of the Institute, was instructed to proceed to Apua and resume 
phototopographic survey work, with a view toward ascertaining 
whether phototopographic surveys would be economical and 
more expedient, now that such decided advances had been made 
in both the manufacture of photographic lenses and in the photo- 
chemical process. 

The practical results of Paganini's investigations and experi- 
mental surveys during the period from 1878 to 1879 were so 
satisfactory that, in 1880, he was directed to begin a systematic 
phototopographic survey of the area boimded by the valleys 
of the Oreo, the Valsoana, the Cogne, and the Valsavaranche, 
comprising an area of about 390 square miles. The survey 
of this mountain complex was finished by Paganini in 1885. 
In 1884, however, the phototheodoUte of the Institute had been 
replaced by an improved instrument of superior qualities, made 
by Galileo in Florence after plans and specifications submitted 
by Paganini and incorporating improvements suggested by the 
experience gained in the field. 

This phototheodoUte, model of 1884, has been fully described 
by Paganini in " La Fototopografia in Italia," Rivista Marit- 
tima, Fasc. VI e VII, 1889; also in Rivista di Topografia e 
Catasto, Nos. 8, 9, e 10, 1889. A German translation, by 
A. Schepp, of L. P. Paganini's " La Fototopografia in ItaHa " 
may be found in the Zeitschrift fuer Vermessung, Nos. 3 and 
12, 1891, and No. 3, 1892. A translated extract from Paga- 
nini's article has been published in Appendix No. 3, in the 
Superintendent's Report of the U. S. Coast and Geodetic Sur- 
vey for 1893. 

Paganini's excellent results effectively estabhshed the effi- 
ciency of phototopography for alpine topography and fully 



26 PHOTOTOPOGRAPHIC METHODS AMD INSTRUMENTS. 

solved the technical side of the problem. Owing to the untiring 
efforts of the officers of the ^Military Geographic Institute toward 
improving phototopographic methods and instruments, the sur- 
veying camera has been adopted as an auxiliary instrument 
to the plane table, the combined use of both instruments in 
the new topographic sur^-ey of Italy having produced the best 
results. 

The latest improvements to Paganini's camera-theodolite were 
first described ia a report to the First Geographic Congress in 
Italy. A German extract from that report by Fenner may 
be foimd in the Zeitschrift fuer Vermessung, 1893. 

The principal departure from the older model (1884) consists 
in abolishing the excentrically mounted telescope and converting 
the camera itseM into a centrally moxmted telescope by replacing 
the ground-glass plate of the camera with an opaque plate having a 
Ramsden eyepiece in its center whenever obsenations with the 
telescope are to be made. This new model (1890), having all the 
details of a theodolite with vertical circle, may be used, when- 
ever the necessity arises, for making angular measurements, 
ia the same way as with an engineer's transit, simply by exchang- 
ing the ground glass for the plate Dyith eyepiece, as just men- 
tioned. 

This instrument, together with the "photographic azimuth 
apparatus " designed for hydrographic surveys, has been described 
by Paganini ia "Nuovi Appunti di Fototopografia; Application! 
della Fotogrammetria all' Idrografia, seguiti alia Nota; La 
Fototopografia in Itaha." Publicata neUa Ri\'ista ^Nlarittima, 
Roma, E. C. Forzani, 1894. 

When the Mihtary Geographic Institute, in 1891, sent some 
map specimens and phototopographic instruments to the Ninth 
Geographic Congress in Menna, in illustration of the Italian 
phototopographic methods. Col. Robert von Stemeck wrote 
to the Institute, in the name of the Committee on Awards, 
that the Italian phototopographic exhibit imdoubtedly desened 
the first prize. Franz Hafferl, Engineer of Austrian Railways,. 



PHOTOGRAPHIC SURVEYING IN SPAIN. 27 

wrote : " Votre exposition photogramm^trique est sans comparison 
la meilleure. Toutes les autres ne sont que des essais plus ou 
moins manques de construction d'appareils phototopographiques 
et des constructions de cartes d'une petite entendue." Dr. S. 
Finsterwalder (Professor of Mathematics and Photogrammetry in 
Munich, Bavaria), Vincenz Pollack (Engineer in Chief of the 
Austrian railroad system), and Col. Otto Krifka (of the Geographic 
Institute in Vienna) also made commendable reference to the 
Italian exhibit. 

Other publications having reference to the phototopographic 
work in Italy, besides those already referred to in the preceding 
paragraphs, may be cited as follows : 

Giuseppe Bertelli. "Noteed AppuntidiTopografia-Fotografia." Rivista 
Militare Italiana, Feb., 1884. 

Capt. Carlo Maeselli. "La Foto-topografia applicata alia Construzione 
della Carta Alpine.'' BoUetino del Club Alpino Italiano, XXIV, No. 57 
1890. 

GlACOMO BuoNOME. "La Foto-topografia in Africa." BoUetino della 
Societa Africana, I e II, 1890. 

Prof. Innocenzo Golfaeelli. "BoUetino della Societa Fotografica Ital- 
iana.'' Firenze, April and May, 1890. 

L. Bennati. "La Fotografia nelle sue Applicazione militare." Rivista 
d'Artiglieria et Genio, II, 1892. 

Col. A. Latjssedat. "Iconomftrie et M^trophotographie Notice sur I'His- 
toire des Applications de la Perspective a la Topographie et a. la Carto- 
graphic." Paris, Photographe, Sept. et Oct., 1891. 

Dr. S. Finsterwalder. "Die Photogrammetrie in den Italienischen Hoch- 
Alpen.'' Mittheilungen des deutschen und oesterreichischen Alpen- 
Vereins, No. i, 1890. 

Wochenschrift des Oester. Ing. u. Archit. Vereins, Nos. 21 and 22, 1890. 



VII. Photographic Survejdng in Spain. 

Although an early interest was manifested in photography 
applied to surveying in Spain, little has been accomplished in 
the practical application of photogrammetrie methods imtil quite 
recently. 



28 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The Madrid Academy of Sciences, in 1862, offered a prize for 
the best treatise in answer to the query, What is the best process 
or method for applying photography to the plotting of maps and 
plans? Of the answers received the memoir, submitted by Capt. 
A. Laussedat in 1863, was awarded the prize. This memorandum 
was accompained by the plan of the village Buc, near Versailles, 
plotted on 1:1000 scale, together with eight photographic views 
on which the topography of this plan was based. 

In the following year Lieut.-Col. Don Pedro de Zea was com- 
missioned to study the French methods and instruments used in 
photographic surveying. Lieut.-Col. P. de Zea examined Capt. 
Laussedat's phototopographic camera (made by Brunner, Paris), 
Capt. ChevaUier's "planchette photographique," Sutton's "pano- 
ramic camera " (made by Thos. Ross, London), etc., collating 
and describing the results of his investigations in : 

Don Pedro de Zea. "Las Applica,ciones de la Fotografia al Service militar." 
Madrid, 1863. 

Don Juan Pie y AUue, Mining Engineer, published a phamplet 
on photo topography, in 1896, in which a specimen survey that he 
had made in the province of Almeria, plotted on 1:1000 scale, 
was included : 

Don Juan Pie yAilue. "Fotogrametria 6 Topografia fotografica." Enrique 
Teodoro. Madrid, 1896. 

A very complete and general work on phototopographic 
methods, instruments, and executed surveys, including experi- 
mental survey specimens made in Spain, has been published by 
Messrs. C. de Iriarte and L. Navarro, in 1899: 

Ceriaco de Iriarte y Leandro Navarro. "Topografia fotografica 6 sea 
Applicacion de la Fotografia al Levantamiento de Pianos." Raoul 
P^ant. Madrid, 1899. 



fHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA. 29 



VIII. Photographic Surveying in the Dominion of Canada and 

in Alaska. 

Capt. E. Deville, Surveyor-General of Dominion Lands, inau- 
gurated extensive phototopographic surveys in Canada, which 
from their inception, in 1888, were marked with great success. 
These surveys were carried out under the auspices of the Canadian 
Department of the Interior in the vicinity of the Canadian Pacific 
R.R. in the Rocky Mountains. A special triangulation had been 
made and a single photographic surveying party of four men 
(imder J. J. MacArthur) covered an average area bf 500 square 
miles per annum until 1892. The winter months of each year 
were spent in Ottawa with plotting the photographic data col- 
lated in the preceding season (under the direction of Capt. Deville) 
on a scale of i : 20000. 

At the World's Columbian Exposition in Chicago, 1893, a 
phototopographic map of a part of the Rocky Mountain Park, 
comprising a dozen sheets of about sixty square miles each, 
published on a i : 40000 scale, formed one of the most interesting 
exhibits of the government of the Dominion of Canada. The 
topography on each sheet was obtained, on an average, from six- 
teen stations, giving from seventy to one hundred and twenty 
panorama views. Six complete panoramas were taken, on an 
average, from stations situated within the limits of the topography 
mapped on each sheet and the development of the terrene was 
controlled by about ten additional camera stations falling outside 
of the actual sheet margin and furnishing ten additional partial 
panoramas. 

From fifteen to twenty points per square mile were plotted 
iconometrically, and whenever possible such points were checked 
by means of vertical and horizontal angles, observed from the 
several camera stations, for locating (instrumentally) a series 
of so-called reference points. These fifteen to twenty points 
form the control per square mile of topography, all intervening 



^O PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

details and topographic features being sketched, after a careful 
study of the panorama views, in a similar manner as the plane- 
tabler would sketch such details, xmder a careful and critical 
study of the surrounding terrene. The published map, scale 
1:40000, shows horizontal contours of one hundred feet vertical 
interval. The average cost of this survey was about seven 
dollars and one half per square mile. 

The atmospheric conditions of this sparsely settled region, 
between the 51st and the 52d degree northern latitude, are notori- 
ously imfavorable for executing the field work of the ordinary 
topographic surveying methods, and the periods of reasonably 
clear weather at best are of short duration. The field season 
lasts about three months — ^July, August, and September — ^and 
even during that short period the observers have to contend with 
fogs, rain, dense smoke caused by forest fires, and snow-storms 
(in the higher altitudes). These conditions being well known, 
Capt. Deville's suggestion, to give the phototopographic method 
a practical trial for the survey of the Rocky Mountain region, 
was endorsed by the Canadian Government. The good results 
obtained in the first season showed that the economical and 
rapid solution of this difficult problem would not have been 
possible without the aid of photography. 

The remarkably good results that were obtained in the photo- 
topographic survey of the Rocky Mountain regions are in a 
'great measure due to the ability of the field observers adapting 
themselves readily to the new methods, but the credit for the 
inception of the work, devising new methods, and a compact and 
serviceable topographic surveying camera suiting the prevailing 
conditions of the country, and for the general excellence of the 
results that were obtained, primarily belongs to Capt. E. Deville, 
Surveyor- General of Dominion Lands and author of an excellent 
manual on " Photographic Surveying," pubHshed by the Cana- 
dian Government at Ottawa in 1889. This edition, of about 
fifty copies, was Hthographed in the Survey's office, having 
been prepared for the use of the Dominion land surveyors 



PHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA. 3 1 

employed under thfe Department of the Interior for making 
the phototopographic surveys. 

The Rocky Mountain work was suspended when the ques- 
tion arose of making a topographic reconnaissance of S.E. Alaska 
for the delimitation of the boimdary line between S.E. Alaska 
and British Columbia. This topographic reconnaissance work 
in Alaska gave the phototopographers of Canada (who for these 
new duties were placed under the direction of Dr. W. F. King, 
Alaskan Boundary Commissioner to H. M.) another opportu- 
nity to demonstrate the superiority of this method above all 
other surveying methods for delineating the topography of a 
country peculiarly rich in climatic and topographic difficulties. 

During the summer months (middle of May to end of August) 
of 1893-94 and to a smaller extent in 1895, this method was- 
used for surveying the topography of S.E. Alaska. Each sea- 
son's work was plotted in Ottawa in the following winter on 
1 : 80000 scale with horizontal contours of 250 feet vertical interval. 

The number of phototopographers prior to 1893 ^^s com- 
paratively small in Canada. Seven of the Dominion land sur- 
veyors were given a practical course in phototopography, under 
J. J. MacArthur, in the suburbs of Ottawa, to familiarize thena 
with the methods and instruments devised by Capt^ Deville. In 
May, 1893, these surveyors were placed in charge of the Cana- 
dian phototopographic parties, each chief having assigned him 
one assistant (also a D. L. S.), from four to five general helpers, 
or packers, and one cook. The survey being jointly made by 
both the Canadian and the American Governments, six of the 
Canadian parties were joined by one U. S. Coast and Geodetic 
Survey of&cer with an additional packer for each American. 

During the summer season of 1893 these parties experienced 
an average of but twenty days favorable for carrying 'on the work 
in the moimtains, and the Canadian expert phbtotopographer 
(J. J. MacArthur) occupied about seventeen camera stations 
during that period. He exposed 108 plates, which controlled 
an area of about 11 50 square miles. The other parties, in charge 



32 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

of less experienced observers, averaged from 450 to 500 square 
miles each during the first season. 

The season of 1894 proved more favorable for the work in 
Alaska than the preceding one, both on account of better weather 
(averaging about forty days suitable for work in the moimtains) 
and because the observers were now more experienced in the 
Tountine and requirements of this class of work. Mr. J. J. 
MacArthur covered an area of about 1900 square miles, having 
occupied twenty-four mountain peaks and exposed 275 plates 
during this season, while the other six parties averaged iioo 
square miles each. 

These results, however, should not be placed in the same 
class with the phototopographic survey of the Rocky Moimtain 
Park, as the result aimed at in Alaska was only a topographic 
recoimaissance, based on a narrow coast triangulation which 
also extended inland along the more prominent inlets and rivers. 
This triangulation had been made by the U. S. Coast and Geodetic 
Survey to control the usual strip of coastal topography and to 
form the basis for the hydrographic surveys of the navigable 
waters of S.E. Alaska. 

The members of the Coast and Geodetic Survey, who had 
been attached to Canadian parties in 1893, became familiar 
with the practical operations and apphcations of the photo- 
topographic surveying method, and, in 1894, Dr. T. C. Menden- 
hall. Superintendent Coast and Geodetic Survey and American 
Boundary Commissioner, had a surveying camera used in con- 
junction with the plane table for the topographic reconnaissance 
at the Head of Lynn Canal, Alaska, by which means the area 
covered with the plane table alone was doubled by the sub- 
sequent iconometric plotting in the office from ninety negatives. 

The same surveying camera was used by the Coast and 
Geodetic Survey parties in Alaska in 1895 (Portland Canal) and 
again in 1897 (Pribilof Islands), under Gen. W. W. Duffield 
Superintendent U. S. C. and G. S. and American Boundary 
Commissioner. 



PHOTOGRAPHIC SURVEYING IN CANADA AND IN ALASKA. 35 

Photography has also been applied recently to surveys made 
for the solution of questions of irrigation in those regions of the 
British N.W. Territories where the rainfall is insufficient for 
agricultural purposes. 

Capt. Deville's first edition of his book on photographic 
surveying having been too limited to supply a general demand 
he yielded to the pressing demand for an English manual on 
this subject by revising and reissuing his book. The valuable 
contents of this work, including the elements of descriptive 
geometry and perspective, fully justify the expectations that 
were connected with its appearance. 

LITERATURE (English). 

Bkidges-Lee. "Phototheodolite," 2d edition. L. Casella, Maker to the 

Admiralty, Ordnance, etc. London, E.C. 
R. Strachy and G. M. Whipple, Supt. "Cloud Photography conducted 

under the Meteorological Council at Kew Observatory." Proc. of the 

Royal Soc. of London, Vol. 49, 1891. 
C. W. Verner. "Notes on Military Topography." Allen. London, 1891^ 
Stanley. "Photographic Surveying." San. Eng., 1892, p. 71. 
Col. A. Laussedat. "Topographical Reconnaissances with the Aid of 

Photography." Phot. Times Almanac, 1895. 
B. J. Edwards. "Color Screens for Use with Isochromatic Plates and 

Films." Phot. Times Almanac, 1895. 
Albert Gleaves. "Some Scientific AppHcations of Photography." Phot. 

Times Almanac, 1895. 
Otto J. Klotz. "Experimental Application of the Phototopographical 

Method of Surveying to the Baird Glacier, Alaska." The University of. 

Chicago Press, 1895. 

The Canadian phototopographic methods have been de- 
scribed in the following publications : 

Capt. E. Deville. "Photographic Surveying, including the Elements of 
Descriptive Geometry and Perspective." Ottawa, Government Printing 
Bureau, 1895. 

Otto J. Klotz. "Photogrammetrische Arbeiten in Canada." Zeitschr. des 
Oester. Ing. u. Archit. Vereins, 1894, pp. 233-234. 



34 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Col. A. Laussedat. "Sur le Progrfes de I'Art de kver les Plans k I'Aide 
de la Photographie, en Europe et en Am&ique." Comptes rendus de 
l'Acad6nie des Sciences, 1893. 

Col. A. Laussedat. "Reconnaissance faite a I'Aide de la Photographie, pour 
la Delimitation de la Frontifere entre 1' Alaska et la Colombie Britanique." 
Comptes rendus de I'Acad^mie des Sciences. Paris, 1894. 

Report of the Superintendent U. S. Coast and Geodetic Survey for 1897, 
Appendix No. 10, '' Phototopographic Methods and Instruments." 
Washington, D. C. 



CHAPTER II. 

THE ELEMENTS OF PERSPECTIVE (CENTRAL PROJECTION). 

Photogrammetry being the inverse of perspective it may 
not be out of place here to review, at least in a summary manner, 
the principal laws of monocular vision, as they are identical 
in a great measure with the laws which form the foundation 
•of perspective. J. H. Lambert (1728 to 1777) apparently was 
the first to lay down rules for finding the point of view of a per- 
spective and to determine the dimensions of objects represented 
in perspective. 



Visual Seeing. 



» 



The eye in order to see an object must receive visual rays 
from every illuminated point of the object. It is a well-known 
fact that the retina of the eye receives an inverted image of every 
sighted object, and yet we all know that objects are seen in 
their natural positions, without requiring a mental transposition 
of the inverted image into the erect position. The explanation 
for this may be found in the so-ca.lled " law of visible direction," 
which, according to LeConte, may be stated thus : " The impres- 
sion on the retina of the eye produced by a ray of light enter- 
ing the eye is referred from the eye along the ray-line back kgain 
into space whence it emanated, and therefore back to its source 
or proper place." 

Every luminous impact which the retina receives by a light 
ray passing through the nodal point of the lens into the eye^ 
is immediately and intuitively referred outward, along the same 

35 



36 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

path which the entering ray traversed, to the true place which 
the luminous point occupies in space. In other words, objects 
sighted as such in space are always the results of " outward 
projections " from the images on the retina through the nodal 
point of the eye as center. 



II. Central Projection. 

If we project from a fixed center — say from the nodal point 
of the eye — the " visible " parts of an object upon a plane inter- 
posed between the eye and the object, the outlines of such pro- 
jection will produce the same impression on the retina of the 
eye as the outlines of the natural object,- provided, of course, 
that one eye only was used and at the same time that the rays 
which emanated from the different points of the pictured object 
could be made of the same kind (of the same intensity and color) 
as those coming from the corresponding points of the object 
itself. The view of such a perspective would then produce sen- 
sibly the same impression on the eye of the observer as the object 
in nature. 

Of the different perspectives capable of being represented 
on a plane surface we are interested here mainly in the so-called 
monocular or focal and linear or mathematical perspectives, 
both outgrowths of descriptive geometry and consisting in the 
application of the rules of projection in general and those of 
orthogonal projection in particular. 

III. Photographic Perspectives. 

The iconometrical problem to be solved in phototopography 
may be stated in the following general terms. From a given 
perspective (central projection) of a body, projected from a 
fixed center (point of view, nodal point) upon a (vertical) pic- 
ture plane, we are to construct the horizontal orthogonal pro- 
jection of that body. 



PHOTOGRAPHIC PERSPECTIVES. 37 

With reference to Fig. i, Plate I, we may say the perspective 
a, b, c, d, e in the vertical picture plane VV is the central pro- 
jection of the object A, B, C, D, E in space from the nodal 
point O as center or point of view. 

Any one object will produce in a given picture plane but one 
perspective image from the same point as center. A point B 
of the object is pictured but once, in 6, 6 being that point in the 
picture plane VV where the visual or projecting ray of the point B 
penetrates the plane VV. 

Of the numerous methods by means of which perspective 
views may be constructed we shall refer only to those which 
have reference to iconometric plotting. To elucidate the close 
connection between the three elements that control or define 
the central projection or perspective, viz., object, picture plane, 
and center of projection, with reference to phototopography 
we may premise, with reference to Fig. 2, Plate I: 

A — The picture plane VV (photographic plate) is sup- 
posed to be vertical. 
B — Through the center of projection O (eye-point) a 

horizontal plane HH is placed (" horizon plane"). 
C — A vertical plane is laid through the center O, inter- 
secting the picture plane at right angles in the line w; 
it is the so-called " principal plane." 
D — ^A plane GG (" ground plane ") is placed parallel 
with the horizon plane HH, but falls below it; the 
distance 00 between the two planes is equal to the 
elevation of the point of view (in the horizon plane) 
above the datum plane (to which all elevations of 
the survey are referred). The ground or datum 
plane in iconometric plotting is identical with the 
plan and it is represented by the surface of the paper 
upon which the topographic map is being plotted. 
The line of intersection g^, Fig. i, Plate I, of the ground plane 
GG with the picture plane VV is known as the " ground line " 
of the perspective. 



38 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The line of intersection hh, Fig. i, Plate I, of the horizon 
plane HH with the picture plane VV is termed the " horizon 
line " of the perspective. 

The line of intersection w of the plane (" principal plane ") 
passing through O and intersecting the vertical picture plane VV 
at right angles, Fig. 2, Plate I, is called the " principal line " 
of the perspective. 

The intersection 0' of the two lines hh and vv, Fig. 2, Plate I, 
is the " principal point " of the perspective. It marks the point 
of penetration in the picture plane of the " principal ray " 00'. 
The principal ray is drawn from the center O (point of view 
or nodal point) horizontally to intersect the picture plane VV 
at right angles. 

The point 0\ where the vertical through the station O pierces 
the ground plane GG is termed the " foot of the station." 

The length of the principal ray 00', equal to the vertical 
distance of the point of view O from the picture plane, is termed 
the "distance Une." 

When the point of view coincides with the second nodal point 
of a camera-lens this same line, the distance line, is known as 
the " focal length" of the camera. 

The perspective view a of a luminous point, A, Fig. 2, PI. I, 
in the vertical picture plane VV is identical with the point of pene- 
tration of the visual ray OA, passing from the luminous point A 
to the center of projection O (point of view or nodal point). 

If we have several parallel vertical picture planes VV, VV, 
V"V", . . . , Fig. 3, PL II, the impression produced on the retina 
of the eye at will remain unchanged, no matter which plane 
VV oi the series may be retained in its position whUe the others 
are removed. 

All planes VV, V"V", ... , placed parallel to the picture 
plane VV are termed "front planes" and any line drawn in a 
front plane will be parallel to the picture plane and is called a 
"front Ime." Front planes may be placed either before or behind 
the picture plane. 



PHOTOGRAPHIC PERSPECTIVES. 39 

The perspective view ab of a line .45 is found in the vertical 
picture plane VV, Fig. 4, PL II, by joining the perspectives a and b 
of its end-points. The perspective ab of a line AB coincides 
with the trace produced in the vertical picture plane FF by a plane 
{so-called "visual plane") passing through O and AB; it is the 
intersection of these two planes. 

The perspective a, b, c, d, eoi a. curve A, B, C, D, E is found 
by locating the perspectives of a series of its points, a, b, c, d, e, 
Fig. I, PI. I, in the vertical plane VV and drawing a continuous 
curve through these points. The perspective of a curve may 
also be obtained by locating the perspectives of a series of tangen- 
tial lines enveloping the curve. The perspective of a curve 
a, b, c, d, e is the intersection with the picture plane of that conical 
visual plane which contains the curve A, B, C, D, E as trace 
and which has its apex in O. 

To draw the perspectives of the superficial planes of bodies, 
the figures inclosing the same (f.i., the perspective of the pentagon 
A, B, C, D, E, Fig. 4, PI. II) are drawn in perspective by con- 
structing the central projections of their perimeters. 

The perspectives of parallel lines when produced will inter- 
sect each other iri one point, the so-called "vanishing point." 

The perspectives of aU horizontal lines {AB and A'B', Fig. 5, 
PI. Ill) have their vanishing point {D) on the horizon line hh 
in the picture plane VV. 

Lines perpendicular to the picture plane have the principal 
point of the perspective as vanishing point (in the picture plane). 

Horizontal lines intersecting the picture plane imder an angle 
of 45° vanish in the so-called "distance points" on the horizon 
line, one on either side of the principal point. Their distances 
from the principal point are equal to the distance line of the per- 
spective. 

The so-called upper and lower distance points are the vanishing 
points for lines falling within the principal plane or that are 
parallel with it and which intersect the picture plane under an 
angle of 45°. The distances of these two points from the principal 



40 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

point are likewise equal to the distance line of the perspec- 
tive. 

Lines parallel with the picture plane, lines in front planes, 
have no vanishing points in the picture plane. Their perspectives- - 
are lines parallel to the original lines. 

Vertical lines (are parallel to the picture plane in our case) 
have no vanishing points and their perspectives are parallel with 
the principal line vv, Fig. 5, PL III. 

Horizontal lines when parallel with the picture plane have 
perspectives that are parallel with the horizon line. 

The scale of a front plane is the proportion between the per- 
spective and the original. It is expressed by the ratio between 
the distances from the station (point of view) to the picture plane, 
the distance line, and that to the figure's front plane (the plane 
containing the original figure). 

The relationship between object (prism ABCD-A'B'C'D'), 
picture plane (FF), and ground plane {GG) may be shown more 
clearly with reference to Fig. 5, PI. Ill: 

O is the station, eye-point, point of view, nodal point, etc. 
A vertical line passing through O will intersect the ground plane 
in Oi. The point Oi is the orthogonal (vertical) projection in 
horizontal plan of the station O and it is called the "foot of the 
station" O. 

The perspective ax of a point Ax', situated in the ground plane 
GG, is obtained by joining the foot of the station Ox with the point 
4 1', erecting a perpendicular to the ground plane in the point of 
intersection ax' of OxAx' with gg and joining O with Ax'. The 
intersection of the ray OA x' with the vertical axax' just mentioned 
will be the perspective of the point Ax' of the ground plane GG. 
A Ax' being a vertical line in space, its perspective aax will be 
parallel with the line w, and, if we draw the ray OA, its inter- 
section a with the line drawn parallel to w through ax, previously 
found, will be the perspective of A. 

To find the perspective of a line AB, the perspectives a and 6- 
of its terminal points A and B may be located in FF and joined 



PHOTOGRAPHIC PERSPECTIVES. 41 

by a straight line ab. Frequently it will be more convenient, 
however, to use the intersection T of the line AB with the picture 
plane VV together with the vanishing point D of the line ab to 
locate the perspective ab of the line AB. This vanishing point D 
is the intersection with the picture plane FF of a line drawn 
through the station O and parallel with the line AB. li AB is 
horizontal, the line OD will fall within the horizon plane and 
intersect the horizon line hh at D. 

The line TT', which is the trace in the picture plane VV of 
the plane ABA'B', is termed the "vanishing line "of the plane 
ABA'B'. 



CHAPTER m. 
PINHOLE PHOTOGRAPHY. 

The photographic camera produces perspectives upon the 
photographic plate through the chemical action of the light 
rays upon the sensitized fihn, and to estabhsh the conditions 
that are to be fulfilled, in order to regard a photograph as a true 
perspective, we will first consider the so-called " pinhole pic- 
tures," which are produced by a camera of the simplest form. 

The pinhole camera consists of a box made entirely Ught- 
tight with the exception of a minute hole O, Fig. 6, Plate IV, 
in the front wall of the box. The rear side of the box is remov- 
able and may be replaced by either a photographic plate-holder 
or a groimd-glass plate. With such a " camera obscura " photo- 
graphs may be obtained without a lens or optical apparatus, 
simply by means of the small round aperture O in the thin front 
wall of the box. 



I. Diameter of the Pinhole. 

When exercising some care, the pinhole may be made by 
burning it into a thin blackened cardboard with a needle heated 
to red heat. The following table gives the diameter in inches 
that may thus be burnt into the cardboard with needles of differ- 
ent sizes: 

Commercial number of sewing-needle 3 6 8 g 

Diameter of burnt hole in inches 1/26 1/34 1/44 1/40 

42 



LENGTH OF EXPOSURE FOR PINHOLE CAMERAS. 



43 



The best results, however, have been obtained with a round 
hole carefully drilled into a sheet of copper or brass 0.2 mm. 
thick. The border of the hole should be perfectly smooth, with- 
out " burr," and it should be beveled that the hole forms a 
truncated cone, the larger circle or base of the cone to face the 
sensitized plate in the camera-box. 



II. Length of Exposure. 

The following table, pubhshed by F. C. Lambert, gives the 
corresponding exposures, in minutes, for pinhole-camera expos- 
ures, if, with the same plate-brand, identical illumination, same 
subject, and a lens working at //16, the correct exposure would 
have been one second. 



Distance of 

Pinhole from 

the Sensitized 


Diameter of Pinhole, in Inches. 


Plate Surface, in 
Inches: 


I /so 


1/44 


I/3S 


I/2S 


6 

8 


6 

10 


4-S. 
8 


5 


— 


10 


16 


13 


8 





12 


24 


18 


12 


6 


14 


32 


24 


15 


8 


16 


40 


32 


20 


10 


18 


— 


41 


26 


13 


20 


— 


— 


32 


16 


24 






~ 


24 



This table plainly indicates that there is little danger of over- 
exposing a plate in the pinhole camera, particularly as these 
exposures are not strictly limited to the time given in the table; 
they depend greatly on the general character of the plate, on the 
developer, and on the general conditions of illumination during 
the exposure, thus giving the operator a wide range regarding the 
time limit of the, exposure. 



44 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



III. Focal Lengths of Pinhole Cameras. 

The depth of focus is practically unlimited, as shown in the 
preceding table of F. C. Lambert. Still, there will always be a 
certain distance between image plane and pinhole that will give 
the best result for a given aperture, and Capt. Colson recom- 
mends the following focal distances for a set of apertures of 
four different sizes: 

Diameter of pinhole in millimeters 0.3 0.4 0.5 0.6 

The best definition is at a focal length, in centimeters ...11 20 30 44 

Using the focal length corresponding to the size of aperture, 
as given in the above table, the time of exposure for a plate in the 
pinhole camera, compared with the exposure required when using 
a lens under identical conditions and with a medium stop, may 
be generally accepted to be: 

25 s° i°° 200 times longer for a diameter of hole of: 

0.3 0.4 0.5 0.6 mm. 

The size of a pictured object, when photographed in a pin- 
hole camera, is proportional to the ratio between the distance 
of the object from the camera and the distance from the pin- 
hole to the sensitized film surface. 



IV. Determination of the Values of the Pinhole-camera Constants. 

It will be a simple matter to determine the values of the con- 
stants of a pinhole camera that are required to be known for 
making iconometric constructions. 

If the angles of the box are exactly 90°, if the, aperture is in 
the point of intersection of the diagonals of the camera front, 
and if means are provided for setting the camera level (for exposing 
the plate in vertical plane), the two lines joining the opposite 



VALUES OF THE PINHOLE- CAMERA CONSTANTS. 45 

middles of the four sides which compose the rear frame of the 
box will represent the horizon line (HH) and the prmcipal 
line {VV) of the photographic perspective. The intersection (P) 
of these two lines will be the principal point and the distance {OP) 
between the aperture and the sensitive film surface will be the 
constant focal length or the distance line of the photographic 
perspective a, b, c, Fig. 6, Plate IV. 

By referring to Fig. 6, Plate IV, it will readily be seen that 
the rays of an object A, B, C, after passing through the aper- 
ture O, produce an inverted image a, b, c on the photographic 
plate. The image obtained in a pinhole camera originates in 
the same way as a perspective is drawn, with the exception 
that the picture plane V'V is not interposed between the eye- 
point O and the original A, B, C, but is here placed behind the 
eye-point, at a distance PO equal to OP', producing an inverted 
and reduced image a, b, c of the original A, B, C. 

By introducing the " negative " with the image a, b, c between 
the eye-point O and the original A,B,CaX V'V, Fig. 6, Plate IV, 
and at a distance from O equal OP' = OP (in- and reverted), 
it would become a " positive." P' being in the prolongation 
of the distance line OP and V'V intersecting the line OP' at 
right angles, the line hh of the " positive " will be horizontal 
and VV vertical. The point a will again be in the point of inter- 
section of the light-ray OA, the point b in the intersection of 
the light-ray OB, and the point c in the intersection of the light- 
ray OC with the plane of the positive V'V. A positive copy 
of a negative will be as true a perspective of the original as the 
negative. Negatives, however, may be used for obtaining any 
measurements that may be required from the perspective for 
the iconometric plotting. Measurements are often preferably 
made on the negatives, as the production of the positives without 
distortion requires considerable care and experience, the amount 
of distortion depending greatly on the character of the material 
on which the positives are made. 

The data given in this chapter may prove useful when a 



46 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

pinhole camera is selected for phototopographic or photogram- 
metric experimental studies in case of an emergency, or when 
the cost of the apparatus must be considered, the pinhole camera 
being recommendable chiefly on accoimt of its cheapness and 
simplicity. 



CHAPTER IV. 

THE FUNDAMENTAL PRINCIPLES OF ICONOMETRIC MAP- 
PLOTTING ("ICONOMETRY"). 

Under " iconometry " we understand the measuring of 
dimensions of objects from their perspective views (" Bildmess- 
kunst ")• It refers to the plotting of terrene forms directly on 
the plotting-sheet from the photographs of the landscape. 

If a photographic perspective of an object, the focal length 
(" distance line "), the second nodal point (" principal point ") 
of the camera-lens, and the horizon hne of the perspective are 
given — if the point of view and the central projection of an object 
are given — these data will be insufficient for the determination 
of the object with reference to position and size. 

If, however, two such perspectives of the same object, obtained 
from two suitably located stations, be given, the dimensions of 
the object and its position with reference to the two stations may 
be determined iconometrically, very much in a manner analogous 
to that in which a point is located (by intersection or by the so- 
called radial method) on the plane-table sheet by being observed 
upon from two known plane-table stations. 



I. Orienting the Picture — Traces on the Plotting-sheet. 

The positions of two camera stations A and A', their linear 
horizontal distance AA' and two photographs, exposed in ver- 
tical plane, one from each station, may be given. Each picture 
may, furthermore, contain the image t of the same object T 
and the image a of the other camera station, Fig. 7, Plate IV. 

47 



48 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

After the base line AA' has been laid down in reduced scale 
4i^i', Fig. 7, Plate IV, and the pictures MN and M'N' are 
brought into the same relative positions with reference to the 
line A\Ax', which they had with reference to the base line A A' 
in the field at the time of their exposure, the position T of the 
point pictured as t and i on the respective pictures MN and 
M'N' may be located (with reference to the line AiAi') by 
drawing the radials Ait and Ai't', when their point of intersec- 
tion will fix the relative position of T with reference to Ai and Ai'. 

The position of T on the map, plotted to scale with refer^ 
ence to the reduced base line or with reference to the plotted 
stations Ai and Ai', would be found by projecting the point 
of intersection T into the plotting or groimd plane. 

A topographic map being the orthogonal projection of the 
terrene forms into horizontal plan, the horizontal projections 
into the plotting-plane of the rays Ait, Aiai', Ai'i, and .4i'd'are 
used to locate the plotted positions of pictured points t arid a 
arid the horizontal projections of the picture planes (which rid'^ir 
become " picture traces ") are utilized in this connection, instead 
of actually using the pictures in the iconometric plotting as' was 
indicated in the diagram of Fig. 7, Plate IV. 

In oider, therefore, to plot the horizontal projection Ti of 
a pictured point t with reference to the plotted base Hne AiAi', 
it ' will become necessary to ascertain the correct positions of 
the picture traces with reference to Ai and A\' — it wiU becdihe 
necessary to "orient" the picture traces M and h'h', Fig. 8', 
Plate V. 

This orientation of the picture traces forms a very important 
part in iconometric plotting, as the subsequent fixing of loca- 
tions of pictured points is accomplished mainly by bringing 
the horizontal projections of their radials (lines of. horizontal 
directions drawn from the different stations to identical terrene 
points) to intersect. Any error in the orientation of the pictui-e 
trace produces corresponding errors in the plotted positions of 
pictured points. 



ORIENTING THE PICTURE — TRACES ON THE PLOTTING -SHEET. 49 

V A. Iconometric Plotting when using a Surveying Camera only. 

A base line measured in the field may have been plotted to 
scale, AiAi, Fig. 8, Plate V, and two pictures, MN and M'N', 
Fig. 9, Plate V, may have been obtained from the camera sta- 
tions A and A' respectively by means of a surveying camera. 
The focal lengths of the pictures =/ and /' respectively, the 
positions of the principal points P and P' and the horizon lines 
HH and H'H' may be known. It is desired to locate Ti with 
reference to the plotted base line AiAi'. 

Wehave4iPi=/; Ai'Pi' = j'; the length of the base =4 1^1', 
and the abscissae tiP=tiPi, ti'P'==ti'Pi', Pai'=Piai', P'ai =Pi'ai, 
Figs. 8 and 9, Plate V. 

The distances Aiai, and Ai'oi, Fig. 8, may be found graph- 
ically by constructing the right-angle triangles AiPiai' and 
Ai'Pi'ai, or they may be computed from 

^lai' ^V (AiPi)^ + {Piai'r, 

These distances are laid off upon AiAi' from A^ and from Ai' 
, respectively a semicircle is described over each length, Aiai 
and Ai'ai, and two circles are drawn about Ai and Ai with 
/ and /' respectively as radii. The intersections of these two 
pairs of circles will locate the horizontal projections Pi and Pi' 
of the principal points P and P' on the two picture traces hh 
and h'h', "the latter being represented by the tangents Piai' 
and Pi'ai. 

B. Plotting the Picture-trace when using a Camera or 
Phototheodolite. 

In this case the angles a and a', Fig. 8, Plate V, may be 
measured directly in the field and plotted on the base line AiAi', 
a at .4i and a' at Ai'. We lay off the distances 



^iai'=V/' + (-P'«i'F 



and ^i'ai=V(/')2 + (Pi'ai)2 



50 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

(found by construction or cpmputation) and describe circles 
about A I and Ai' with / and /' respectively as radii. The 
tangents drawn from ai' and fli to these circles will locate Pi 
and Pi respectively when Pih should equal tiP=x, measured 
on the picture MN, and Pi'h' =P'ti' =x' on M'N'. 

When using a phototheodolite a well-defined point T may 
be bisected with the principal lines VV and V'V, Fig. 9, Plate V, 
from the two stations A and .4i, in which case these angles of 
orientation are laid off upon the base line at ^i and at Ai' re- 
spectively, and the distances / and /' are laid off on the lines 
AiTi and Ai'Ti respectively {=AiPi and =Ai'Pi'), when the 
perpendiculars to AiPi in Pi and to Ai'Pi in Pi' will represent 
the picture traces hh and h'h' in correct orientation with refer- 
ence to Ai, Ai, and Ti. 

When the pictures of several triangulation points B, C, and D 
and the base line are given, the orientation of the picture traces 
hh and h'h' upon the plotting-sheet may be accomplished as 
follows : 

The radials AiBi, AiCi, AiDi . . . , as well as the radials 
Ai'Bi, Ai'Ci, Ai'Di . . . , are drawn upon the iconometric plot- 
ting-sheet, the points Bi, Ci, Di . . . being already plotted on 
the same. The points hi, Ci, P, di, and ai' are then transferred 
from the horizon Hne OOi of the photographic perspective MN, 
Fig. II, Plate VI, upon the perfectly straight edge of a strip 
of paper, which now is placed upon the radials converging to Ai, 
as a center, Fig. 10, Plate V, and moved about imtil 

bi falls upon the radial line AiBi, 

ci " " " " " AiCi, 

di " " " " " AiDi, 

ai' " " " base " AiAi'. 

The line AiPi should then be perpendicular to the straight 
edge hh of the paper strip. Fig. 10, Plate V, and the line hh, 
drawn along the paper strip's edge on the plotting-sheet will 



DETERMINATION OF PRINCIPAL. AND HORIZON LINES. 5 1 

represent the oriented picture trace of MN; AyPi will be the 
distance line and Pi the horizontal ;pr9Jection of the principal 
point p. 

The same having, been done regarding, the point Ai', its 
picture M'N' and the paper strip OiOi' (Fig. ii, Plate VI), 
both picture traces hh and h'h' will have been oriented. The 
plotted positions of any other pictured points that may be iden- 
tified on both pictures MN and M'N' may be similarly located 
by plotting their abscissae (measured on the horizon lines OOi 
and O'Oi) upon the picture traces hh and h'h' (Fig. lo, Plate V) 
on the proper sides of the principal points Pi and Pi'. 

Lines drawn from the station points AiAi' through such 
corresponding points, transferred to their respective picture 
traces, will locate the relative positions of such points on the 
plotting-sheet by their points of intersection. 



II. Arithmetical Determination of the Principal and of the 
Horizon Line on the Photographic Perspectives. 

In the preceding paragraphs it has been assumed that the 
photographic perspectives were already provided with the prin- 
cipal and the horizon lines. Such, in point of fact, would be 
the case with an adjusted surveying camera or phototheodolite. 
If the instrument is out of adjustment or if an ordinary camera 
"be used (one provided with a device for maintaining the image 
plane in a vertical position during the exposure of the plate), 
the correct positions of the principal and horizon lines, as well 
as the length of the distance line, must be ascertained. In photo- 
topographic work this may be accomplished in various ways. 

A. Determination of the Principal Point and Distance Line of 
the Photographic Perspective. 

A plumb line suspended in front of the camera in such a 
way that the line vv, Fig. 12, Plate VI, may be photographed 



52 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

upon the negative will serve to establish the direction of the 
principal line on the trial plate. This negative may, further- 
more, contain the images a, 6, c ... of three or more points A, 
B, C, . . . oi known positions and elevations. A line hh is drawn 
at right angles to the pictured plumb line vv on the photographic 
perspective and a strip of paper is placed with its straight edge 
along this line. The images a, b, c ... of the known points 

A, B, C... are projected upon the paper straight edge, held 
in position at hh, by drawing parallels to vv through these pic- 
tured points. 

After the radials from the plotted station 5i, Fig. 12, Plate VI, 
have been drawn through the plotted points Ai, Bi, Ci . . . 
the paper strip is adjusted upon those radials in such manner 
that the image projections ai, 61, ci . . . (previously marked 
on the strip) will fall upon their corresponding radials; a line 
drawn along the edge of the paper strip while in this position 
will represent the oriented picture trace, as indicated by the 
line hihi. 

If we now draw a perpendicular line {S\Pi) to hihi from the 
plqtted station Si, the point P\ will be the horizontal projection 
of the principal point P and SiP\=j will be the distance line 
for the perspective MN. 

Should the positions of the points A, B, C . . . with refer- 
ence to the station S be not known, it will become necessary 
to observe the horizontal angles ASB, BSC, CSD . . . instru- 
mentally from the station 5 and plot them in their proper order 
upon a sheet of paper (AiSiBi, BiSiCi . . . ) and adjust the 
paper strip hh upon these radial^ in the same manner as just 
described. 

B. Determination 0} the Position of the Horizon Line on the 

Perspective. 

When the elevations AA', BB', CC . . . (Fig. 13, Plate VII) 
of the points A, B, C . . . above the horizon plane SOO' of the 



DETERMINATION OF PRINCIPAL AND HORIZON LINES. 53 

Station S are known, the position of the horizon line 00' on 
the perspective MN may be found by computing the ordinates 
aa', bf, cc^ ... from the equations : 

aa':AA'=Sa':SA', 
bb':BB'=Sb':SB', 



whence 



bb' = 



Sa'xAA' 

SA' 
Sb'xBB' 



<^'=—SA^=y' 



SB' 



-yx 



The distances So!, Sb', Sc' . . . are taken from the plotting- 
sheet. The horizontal distances SA', SB', SC . . . and the 
differences in elevations AA', BB', ^CC . . . are known. 

For example, the difference in elevation between A and 
4' = 100 m., the distance of A' from the station 5 = 1000 m., 
and the distance Sa', measured on the plotting-^heet, =0.05 m., 
then we will have 

, 0.05 X 100 

aa'=y = =0.00? m. 

"^ 1000 '^ 

The horizon line 00' on the negative will be 5 mm. ver- 
tically below the pictured point a (measured in a direction parallel 
with the pictured plumb line w). A line 00' drawn through a' 
at right angles with the pictured plumb line vv will locate the 
horizon line. The computed ordinates bb'=yi, cc'=y2...oi 
the other pictured points b, c . . . will serve to checTi the position 
of the horizon line 00' ; it should be tangent to the arcs described 
with aa', bb', cc' . . . about a, b, c . . . respectively as centers. 



54 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



ni. Graphic Method for Determining the Positions of the Prin- 
cipal and Horizon Lines on the Perspectives. 

The following method for orienting the picture trace, pub- 
lished by Prof. F. Schiffner, in 1887, and mentioned by Prof. 
Steiner, leads to the same result graphically as the precedmg 
one does arithmetically. 

The horizontal projections Ai, Bi, Ci, and Si of three points 
A, B, C, and station S, Fig. 14, Plate VTI, may be given. From 
Si, as center, radials are drawn through ^1, Bi, and Cj. Through 
a point a on the radial SiAi a parallel to SiCi is drawn and 
the distance a'l/ — taken from the negative MN, not shown 
in the figure — is laid off from a =061' upon this parallel line, 
while the distance b'c' is laid off upon the same line from 6/ 
= bi'ci'. 

Parallels to the radial SiAi are then drawn through the 
points bi' and ci' and produced to intersect with the radials 
SiBi and StCi. The line h'h' connecting these two points 
of intersection will be parallel with the direction of the picture 
trace. 

The same distances a'b' and 6V — taken from the negative — 
are laid off upon this line h'h' from a2 = fl^2&2 and from 62 = ^2^2 
respectively. The parallels to the radial SiAi, drawn through 
these points 62 and C2, are brought to intersections with the radials 
SiBi and SiCi, when the line hh, passing through these inter- 
sections b' and c', will represent the picture trace, oriented with 
reference to Si, Ai, Bi, and Ci. 

The distance SiPi of Si from hh represents the distance line 
(focal length) of the picture MN, and the point Pi will be the 
horizontal projection of the principal point of the perspective. 

After having transferred Pi (with reference to a', b', and cf) 
to the perspective MN by means of a strip of paper, a parallel 
to the pictured plumb hne w drawn through the point Pi will 
locate the principal line upon the negative. 



THE " FIVE-POINT PROBLEM. 55 

III. The " Five-point Problem " (by Prof. F. Steiner), or Locating 
the Plotted Position of the Camera Station by Means of the 
Perspective when Five Triangulation Points are Pictured 
on the Same Photographic Perspective. 

In the methods considered until now it had been assumed 
that the position of the camera station Si on the plotting-sheet 
was known with reference to the plotted triangulation points 
-4i, Bi, Ci . . . . 

In case the panorama pictures have been taken from a camera 
station Si of unknown position and a series of known points 
are pictured upon the panorama views, both the position of the 
camera station may be found (with reference to the positions 
of the surrounding points of known positions) and the picture 
trace may be oriented by means of Prof. F. Steiner's " five-point 
problem," if one of the panorama views contains the pictures 
of five or more points of known positions. 

A. Determination of the Principal Point and Distance Line. 

A panorama view MN may contain the images a, b, c, d, e 
of the triangulation points A, B, C, D, E, already plotted on 
the working-plan, and also the picture of a suspended plumb 
line or other vertical (or horizontal) line sufliciently long to be 
used for drawing parallel lines to the principal (or horizon) 
line. 

The points a, b, c, d, and e of the negative MN are projected 
upon the straight edge of a strip of paper = ai, 61, ci, di, and Ci. 
Radials are now drawn from one — Ai, Fig. 15, Plate VIII — 
of the five plotted points as center to the other four points, Bi, 
Ci, Di, and £1. The paper strip is then placed over the radials 
AiBi, AiDi, and AiEi, that 61 falls wpon AiBi, di upon AiDi, 
and ei upon AiEi, when the strip will have the position ai, 
*i, ci, di, e\. The line drawn through Ai and a^ (the latter 
having been transferred to the sheet by means of the paper strip) 



56 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

will be tangent in A to the ellipse £1 (which passes through Ai, 
Bi, Pi, and Ei and through the plotted station Si). 

The paper strip is now placed over the radials AiBi, AiCi, 
and AiDi, that bj falls upon AiBi, Ci upon AiCi, and di upon 
AiDi, when the strip will have the position indicated by 02. 
62? C2, d2, ei, and the line A\a-2, will be the tangent in .4i to the 
ellipse £2 (passing through the points A\, B\, C\, Z>i, and the 
plotted station point 5i). 

The plotted position of the station point S\ with reference 
to the five plotted points Ax, Bx, Cx, Dx, and Ex will be at the 
fourth point of intersection 5j of the two ellipses Ex and E2. 

After drawing the radials SxAx, SxBi, SxCi, SxDx, and SxEi 
the paper strip is placed over these radials in such manner that ax 
falls upon SxAi, bx upon SxBx, . . . and ex upon SxEx, in the 
position indicated by a, b, c, d, e=HH, when HH will be the 
plotted picture trace. 

The perpendicular upon HH passing through 5i=5iPi 
represents the distance line and Pi is the principal point of the 
negative projected into the horizontal plan, which, in order 
to locate the principal line, may now be transferred to the per- 
spective by means of the paper strip in the manner already 
described. 



B. Simplified Construction for Locating the Plotted Position of 
the Camera Station by Means of the " Five-point Problem." 

The method just described being rather complicated. Prof. 
Schiffner recommends the following construction, Fig. 16, Plate IX, 
in which the drawing of the two ellipses Ex and E2 is avoided: 

The plotted positions of the same five points A, B, C, D 
and E, together vidth a negative containing the images a, b, c, d, 
and e, of these points may be given. It is desired to find the 
fourth point of intersection Sx of the two ellipses Ex and E2 
without actually drawing their perimeters. 



THE ' FIVE-POINT PROBLEM. 57 

The two tangents baBi and b^Bi to the ellipses Ei and -£2 
in Bi are located in precisely the same manner as the two tan- 
gents aiAi and 02^1 for the point Ai were found in Fig. 15, 
Plate VIII. The intersections i?i and i?2 of the tangent pairs 
aiAi, 63B1, and 02^4 1, bJBi, Fig. 16, Plate IX (belonging respec- 
tively to the ellipses £1 and £2), are situated on a Ime QX, form- 
ing one side of the polar triangle QXT, common to both ellipses. 
This line RiR2=QX intersects the diagonal AiDi in X and 
the quadrilateral side BiDi in the point Q. The lines drawn 
through Q from Ai and through X from Bi will intersect each 
other in the fourth point of intersection Si of the two ellipses. 

This method may also appear rather complicated in view 
of the many lines that have to be drawn before the picture trace 
HH aiid the position of the ca;mera station may be plotted. 

C Application of the " Five- point Problem " to the Special Case, 
where the Five Points range themselves into a Triangle on the 
Working- sheet. 

The application of the five-point problem becomes very 
much simplified when the five points A, B, C, D, and E form 
a triangle of which two sides A^Ci and Ci£i, Fig. 17, Plate X, 
contain three points each. 

If we place the strip of paper upon the radials, drawn from Ai, 
that ci falls upon AiEi, di upon AiDi, and ci upon AiCi, it will 
have the position indicated by 02, ^2) C2, d2, £2, and the first ellipse 
will resolve into the lines Ci£i and ^102- If we riow place the 
paper strip ai, bi, Ci, di, ei upon the radials drawn from £1 to Ai, 
to Bi and to Ci, that ai falls upon £1^1, &i upon EiBi, and' ci 
upon EiCi, it will assume the position ai, bi, Ci, di, ei, and the 
second ellipse will resolve into the lines AiCi and £iei. 

The intersection Si, of the two lines ^102 and £1^1, locates 
the position of the plotted station point Si with reference to the 
five given points Ai, Bi, Ci, Di, £1. By placing the paper 
strip upon the radials SiAi, SiBi, SiCi, SiDi, and 5i£i^ in 



58 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

such manner that ai falls upon SiAibi, upon SiBi, Ci upon 
SiCi . . . , its edge HH will locate the picture trace, Pi' will be 
the horizontal projection of the principal point P, and 5ii'i will 
be the distance line. 



D. To find the Elevation of the Camera Horizon for a Station 
that has been located by means of the "Five- point Problem." 

To ascertain the elevation of the station S, plotted after one 
of the preceding methods, it will be necessary to know the ele- 
vations of at least two of the five given points. The elevation 
of the station horizon SOO', Fig. 13, Plate VII, above the datum 
or ground plane S'OiOi', may be designated by X, H and Hi 
may be the elevations of A and B respectively, both supposed 
to be known. The ordinates of the pictured points a and b 
are aa' =y and bV =y. 

From the relation S'ai' -.S'Ai' =aa' -.AA' 

or Sa':SA'=y:{H-X) 

Sa' 
we find y=-^,{H-X), 

SV 
and yi=^(lZ'i-Z). 

,As the difference between y and yi may be found by direct 
measurements made on the negative, y—yi=m will be known 
and the value for X may be computed from the equation 

y-yi=m = {H-X)j^,-{Hi-X)^, 



since the measures for Sa', SA', SV, and SB' may be obtained 
from the plotting-sheet, measured in the scale of, the, map. 



THE "three-point PROBLEM." 59 

The above equation may be written in the general form: 

H-X H-X ^ I 5a' , I SV 

^-~ J-' -^^^'^ -^=^' ^"d -=^; 

hence 

m-n-p—H-p—Hi-n 

X — 7 . 

n—p 

By substitution of this value in the equations 

H-X , Hi-X 

the numerical values for the ordinates y and yi (governing the 
position of the horizon line) may be found. 



V. The " Three-point Problem," 

If the triangulation points are not sufficiently close together 
that five or more points may be pictured on one photographic 
perspective, and if stations are occupied with the camera that 
are not directly connected with the trigonometric system, it will 
become necessary to employ other means than those hereto- 
fore considered for locating the position of such detached camera 
stations with reference to the triangulation system. 

To connect detached camera stations with the triangulation 
by observations made at the camera station, at least three tri- 
angulation points should be visible from such station. When 
the camera party is in advance of the triangulation party many 
camera stations will be located by the triangulation party by 
observing upon a signal left at the camera station, if such signal 
be visible from two or more triangulation stations (the camera 
station will be a " concluded point " of the triangulation system). 



6o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The determination of the position of a detached camera 
station by observing upon three fixed and known points (pro- 
vided with signals) is generally known as the " three-point prob- 
lem " (station-plotting, station-pointing, etc.), or " Pothenot's 
method," although SneUius was probably the first to use this 
method (in his trigonometric surveys in the Netherlands in the 
second decade of the seventeenth century). 



A. Mechanical Solution of the " Three-point Problem" 
{using a Three-arm Protractor or Station-pointer). 

The simplest solution of the three-point problem is purely 
mechanical in application. The two observed angles M and N 
are laid off upon a three-arm protractor (" station-pointer ") 
or upon a sheet of tracing-paper, and the three arms or lines SiAi, 
SiBi, and SiCi, Fig. i8, Plate X, are placed over the three fixed 
and plotted points Ai, Bi, and Ci in such manner that the three 
lines of direction SiAi, SiBi, SiCi pass through their respective 
points Ai, Bi, and Ci, the point 5 being transferred to the 
working-sheet while holding the two horizontal angles M and N 
in unchanged position. 



B. Graphic Solution of the " Three-point Problem." 

I. Using the So-called "Two-circle Method." 

Theoretically the best graphic method is probably that by 
which the position of the fourth, or station, point is located at 
the intersection of two circles, one passing through Ai and Bi 
and having over AiBi, as chord, the angles of circumference 
=AiSiBi=M, Fig. 1 8, Plate X, the second circle passmg through 
Bi and Ci and having over the chord BiCi the angles of cir- 
cumference equal to BiSiCi=N. 



THE "three-point PROBLEM." 6l 

From the plotted triangle side AiBi we lay off at i4i and Bi 
the angles BiAiCi and AiBiCi, each equal to 

=9o°-/li5i5i = 9o-lf, 

and about the point Ci, thus obtained, we describe a circle 
AiBiSi with the radius =Ci4i=Ci5i. The observed angle 
AiSiBi=M will then be an angle of circumference over AiBi, 
and the point 5i will be located somewhere on the arc over AiBi. 
By means of the angle BiSiC=N another circle BiCSi is 
described over the triangle side BiC, in a similar manner, about 
the point C2 as center, having C2-Bi=C2C as radius. The 
observed second angle BiSiC=N will be an angle of circum- 
ference over the chord BiC and the point 5i will be on the arc 
over BiC hence its true position is at the (second) point of inter- 
section 5i of the two circles. 

2. Using the Method or Bohnenberger and Bessel. 

The following method, by Bohnenberger and Bessel, is readily 
applied and simple in construction. If we describe a circle 
through two of the given points, through Ai and Bi, Fig. 19, 
Plate XI, and through the station 5i, the angles designated by M 
and those designated by iV in the figure will be respectively 
equal, being angles of circumference over the same arcs AiDi 
and DiCi respectively. 

Hence if we lay off the observed horizontal angle N on AiCi 
at ^1, and the other observed horizontal angle M on the line 
AiCi at Ci, the point of intersection Di of their convergent 
sides CiDi and AiDi will fall upon the line connecting the third 
plotted triangulation point Bi with the station point 5i. 

After having thus determined the direction of the line BiDi 
or BiSi the position of the point sought may be found as follows : 

At any point x on the produced line BiDi the observed angles 
M and N are laid off to either side of BiDi, in the sense in which 



62 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

they were observed at S. Lines AiSi and CiSi, drawn through 
Ai and Ci, parallel to xy and xz respectively, will locate the 
plotted position of the station point Si (upon DiBi) with refer- 
ence to the three plotted points Ai, Bi, and C\. 

This solution is recommended only when Bi£>i is sufficiently 
long (in Fig. 19, Plate XI, it evidently is too short) to assure a 
correct prolongation toward Si. 

The picture trace HH, containing the horizontal projections 
of the pictured points a, b, c, may now be oriented in the known 
manner by adjusting the paper strip, having the three points ai, 
hi, and ci marked on its edge, over the radials SiAi, SiBi, and 
SiCi to bring ai on SiAi, bi on SiBi, and Ci on SiCi. 



VI. The Orientation of Picture Traces, Based on Instrumental 
Measurements Made in the Field. 

When no points of the area to be mapped phototopographic- 
ally are known, the elements (horizon line, principal point, and 
distance line) of the photographic perspectives can no longer, 
be determined from the photographs alone. Instrumental obser- 
vations will have to be made at the camera stations in the 
field to supply the data needed for their determination. This 
-method, among others, having been adopted by Capt. Deville, 
will be described in the chapter giving the description of the 
Canadian surveying-camera. 



VII. Relations between Two Perspectives of the Same Object, 
Viewed from Different Stations. 

(Prof. Guido Hauck's Method.) 

A more general application, of photogrammetric methods 

dates from 'the publication of Prof. G. Hauck's investigations 

and results regarding the relationship between trilinear systems 

of different planes , (Guido Hauck, " Theorie der trilinearen 



TWO PERSPECTIVES OF THE SAME OBJECT. 63 

Verwandtschaft ebener Systeme," Journal fuer reine und ange- 
wandte Mathematik, L. Kronecker und Weierstrass, Bd. 95, 
1883). In this publication Prof. Hauck discusses the relation- 
ship between the projections of the same object upon three differ- 
ent planes. The practical value of his theoretical deductions 
was fully established and tested practically by the students of 
the Royal Technical High School of Berlin who attended Prof. 
Hauck's lectures and exercises connected with the course in 
descriptive geometry in 1882. 

In the discussion of the relation between three perspectives 
of the same object Prof. Hauck refers to some properties of 
decided value in iconometric plotting. The principal law (as 
deduced by Prof. Hauck) with reference to phototopography 
may be stated as follows: 

If an object be projected from three different points as cen- 
ters upon three different planes that may have any position in 
space, one of these projections (perspectives) can be evolved 
from the other two by means of graphic construction. Or, 
expressed in terms more suited to our case, if an object has 
been photographed on three plates exposed from different 
stations, any one of these photographic perspectives may be 
evolved graphically from the remaining two. A topographic 
map (the orthogonal projection of the terrene) may be regarded 
as a central projection or perspective in a horizontal plane, 
having its center of projection (point of view) at infinite ,dis^ 
tance, and we may state Prof. Hauck's law as follows: 

From two photographs MN and M'N' of the same terrene, 
taken from different stations 5 and S', the orthogonal horizontal 
projection of the terrene may be obtained graphically by means 
of rays emanating from the so-called " kernel points " (" Kem- 
pimkte ") as centers. 

The line of intersection of the two photographs (the two 
planes of projection or perspective planes) MN and M'N', 
Fig. 20, Plate XI, will be the '" perspective axis." 

To better illustrate' the connection betweep two different 



64 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

photographs we will first refer to the simple case of two vertical 
perspective planes or photographs MN and M'N', Fig. 20, 
Plate XI. 

A. " Kernel Points " and " Kernel Planes." 

Let 5 and 5' represent the two camera stations (centers of 
projection or points of view for the two vertical photographic 
perspectives MN and M'N'), let s' be the picture in MN of S', 
and s be the picture of 5 in M'N', then these two pictured sta- 
tions s and s' will be so-called " kernel points " (" Kempunkte "). 

The two picture planes MN and M'N' intersect each other 
in the line IQ, the so-called " perspective axis." 

Planes passing through the line SS' (base line) will contain 
the " kernel points " s and s'; they are termed " kernel planes " 
<" Kemebenen "). 

A kernel plane M2N2, laid through any point A, pictured 
as ■a and a', will intersect the first picture plane MN in the line as' 
and the second picture plane M'N' in the line sa'. These lines 
of intersection {as' and a's) will intersect the " perspective axis " 
IQ in the same point i2 ; they will contain the pictures a and a' 
of the point A, and they will pass through the picture 5 and s' 
of the two camera stations S and S'. 

The lines S'A, SA, SS', as', and a's fall within the " kernel 
plane " M2N2. All lines as' for all points pictured in MN 
will pass through the pictured station point s' (image of the second 
camera station S'), and all lines as for the picture plane M'N' 
will pass through the pictured point s of the camera station S. 
furthermore, all pairs of lines (as' and a's) joining the per- 
spectives (a and a') of identical points (A) with their corre- 
sponding pictured station points (" kernel points " s' and s) 
will intersect the " perspective axis " (IQ) of the two pcture 
planes (MN and M'N') in identical points (Q). 

From two photographs of the same object which also con- 
tain the pictures of the two reciprocal stations pecuhar advan- 



TWO PERSPECTIVES OF THE SAME OBJECT. 65 

tages may be gained for the iconometric plotting, inasmuch as 
such pictured stations s' and s will be " kernel points." 

The perspective axis of the picture planes may also play an 
important part in iconometric plotting, not only for pictures 
exposed in vertical planes, but even more so for inclined picture 
planes. 

If two photographs MN and M'N' are given (in Fig. 21, 
-Plate XII, their traces are represented by the lines HH and H'H'} 
representing the same object, viewed from the two stations S 
and S' without containing the pictures of the stations, the posi- 
tions of the pictures s and s' of the corresponding camera sta- 
tions 5 and S' may be located upon the picture planes (out- 
side of the actual field of the photograph) by construction. 

The horizontal projections ^i and 5/ of the " kernel points "■ 
s and s' are identical with the points of intersection of the plotted 
base line SS' and the picture traces HH and H'H', Fig. 21, 
Plate XII. Hence, if we revolve the picture planes MN and 
M'N' about their ground lines, until they coincide with the 
ground plane, the line IS, common to both picture planes (the 
" perspective axis "), will be represented by the two lines i(I)y 
and the " kernel points " s and 5' of the revolved planes MN 
and If A/" will fall upon the lines Si(Si) and Si'(Si') respectively 
(these lines are^ perpendiculars upon the picture traces in the 
horizontal projections ^i and ^i' of the kernel points ^ and s'). 

To find the lengths si(Si) and Si' (5i') (the ordinates of 
the " kernel points " in the picture planes above the ground 
lines) we erect perpendiculars to the base liae in 5 and S' with 
lengths equal to the elevations of the camera stations above 
the ground plane = 5(5) and S'(S') respectively. 

The line (5)(5')^the vertical plane passing through the base 
line 5 has been revolved about the horizontal projection of the 
base line into the groimd plane to coincide with the latter — 
will intersect the lines 5i(5i") and Si'(Sii') — they are perpen- 
diculars to SS' in the " kernel points " J and ^ — and the lengths 
■Si('S'i") and ^i'(5ii') will be equal to the ordinates of the " ker- 



66 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

nel points " s and s' above the ground lines of MN and 
M'N'. 

The " kernel points " s and s', Fig. 20, Plate XI, may be 
located after this manner in the picture planes of any two photo- 
graphs, provided such picture planes are not parallel (or even 
nearly so) with the base line SS'. 

B. Use of the "Perspective Axis" (Line of Intersection) of two 
Picture Planes that show identical Objects viewed from 
different Stations. 

If a series of characteristic points of the terrene, pictured 
in a vertical plane MN, Fig. 22, Plate XII, are connected with 
the " kernel point " s by straight lines, these will, when pro- 
duced, intersect the perspective axis IQ, and if the images of 
the corresponding identical points in the vertical picture plane 
M'N' are joined with the " kernel point " s', and if these Unes 
are likewise produced to intersect the " perspective axis " IQ, 
the points of intersection of IQ with the first group of lines (belong- 
ing to MN) will be identical with the points of intersection of 
IQ with the second group of lines (belonging to M'N'). 

If we now provide the " perspective axis " with a scale of 
equal parts (having the zero or origin of graduation in the ground 
plane), lines drawn through the " kernel points " and through 
corresponding images of identical points in both picture planes 
will intersect identical points of this scale. 

The length O'O, Fig. 22, Plate XII, intercepted on the scale 
of the " perspective axis " by the two horizon lines of the picture 
planes MN and M'N' represents the difference in elevation 
of the two camera stations S and S'. The scale IQ may be 
drawn on both pictures to show on both lines i{I), Fig. 21, Plate 
XII, after the pictures have been separated. Frequently the 
picture itself will not be sufl&ciently extended to contain the 
line IQ, in which case such a scale may still be used by placing 
it upon a Une XX", in MN, and upon zz", in M'N', some dis- 



TWO PERSPECTIVES OF THE SAME OBJECT. 67 

tance from but parallel with the perspective axis IQ, Fig. 22, 
Plate XII, provided the following relation remain satisfied : 

sQ:sx' = s'Q:s'z'. 

For any other, point B, photographed as b and b' in the pic- 
ture planes MN and M'N' respectively, the following propor- 
tional equation should be fulfilled: 

sP:sxo=s'fi:s'zo. 

The triangles sxqX^, s^Q and s'zqz', s'^Q being, respectively, 
similar, x^oc" must be equal to z^z' (as /?i2 is common to both 
triangles s^Q and s'^Q), which means the spaces on the scales 
XX" and zz" are to be identical in numerical value. The two 
scales (or either of them) may, if more convenient, be placed 
beyond s or /, f. i. at tt", in which case 

5/? : sto = J/3 : sxq = s'^ .s'zq. 

It should be noted that the scale is now to be read from f toward 
Iq. It may be stated generally that the scales should be placed 
parallel with the " perspective axis " 18 and at distances from the 
" kernel points " directly proportional to the distances of the latter 
from the " perspective axis " of the picture planes, their correct 
position being found from the horizontal projection or from 
the ground plane. To avoid obscurity and obliteration of details 
in the field of the photograph it will generally be more expe- 
dient to draw these scales outside of the picture proper. 

To find the proper position of the second scale on the second 
picture, after the position of the scale on the first picture has 
been decided upon, we again refer to Fig. ai, Plate XII, where 
HH and H'H' are the two picture traces, 5 and S' are the 
horizontal projections of the camera stations, P and P' are the 
traces of the principal lines // and /'/' (Fig. 22), or the horizontal 



68 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

projections of the principal points, and, finally, h the selected 
position for the first scale. 

To find the corresponding position h' of the second scale 
we draw a line hh parallel to SS' through h, when 

s'i:s'h=si'i:si'h'; 
hence 

s'-t 

=distance of the second scale from 
the " kernel point " 5' in the second picture. 

The conditions and relations described in the foregoing 
paragraphs may often prove of value in iconometric plotting; 
f. i., if we consider the case of a straight line L, Fig. 23, Plate XIII, 
the image of which appears in picture MN as /, but in the second 
picture M'N' only a short piece I' is seen. It may be desirable 
to locate in the picture plane MN the reciprocal position of a 
point X, shown on the line / in MN, but falling outside of the 
picture limit of M'N' on the prolongation of /'. 
To find the position of x/ we proceed as follows: 
The pictured point x of the line I, pictured in MN, is con 
nected with the kernel point (s') and the line (s')x is produced 
to its intersection (x) with li. After transferring the point (x) 
to the line il of the second picture plane M'N', to ({x)), and 
connecting the latter with the " kernel point " (s), the intersec- 
tion of ((x))(s) with I' produced will represent the point sought, 
x', on the prolongation of the line /'. 

VIII. To Plot a Figure, Situated in a Horizontal Plane, on the 
Ground Plan by Means of its Perspective. 

Excepting the shore lines of lakes and coasts and the out- 
lines of marshes, figures in horizontal planes are not frequently 
met with in topographic surveys, and the simplest way to map 
these woidd be to expose photographic plates in a horizontal 



TO PLOT A FIGURE BY MEANS OF ITS PERSPECTIVE. 69 

position from a captive balloon at points of known positions 
and at identical or known elevations. 

The mapping of such figures, when photographed on ver- 
tically exposed plates, from stations above the figure's plane 
is also an easy matter. It may even be done with but a single 
perspective view of such figure (obtained on a vertically exposed 
plate from a station of known position), provided we also know 
the difference in elevation between the camera station and the 
horizontal plane containing the figure, and provided we know 
the positions of the principal point and horizon line together 
with the length of the distance line (focal length) of the photo- 
graphic perspective. 

We have, with reference to Fig. 24, Plate XIII, i?iZ'= horizon 
plane of the camera station S, 00'= horizon line of the photo- 
graphic perspective MN, GG= ground plane or horizontal plane 
coinciding with the surface plane of the lake A BCD, SSo = h 
= difference in elevation between the camera station 5 and the 
water level of the lake. 

With a given perspective abed of the lake A BCD in "the ver- 
tical picture plane MN, knovm focal length, given position 
of the principal point P and known difference in elevation, h,. 
between the water surface of the lake and the camera station^ 
the projection of the lake-outline (AiBiCiDi) in horizontal plan 
may be drawn. 

The ground line OqOo' (line of intersection of ground plane 
GG with the vertical picture plane MN) is drawn through Pq, 
(horizontal projection of P) parallel with the horizon line 00', 
PPq being equal to h (measured in the plotting-scale). If we 
now project the pictured points a, b, c, d upon OoOo' = ao, bo, 
Co, do, the radials from the foot So of the station 5 drawn through 
the points ao. ^o. Cq, do, will pass through the corresponding 
points of the lake shore-Une Ai, Bi, Ci, Di that are to be plotted. 

Referring to the vertical plane passing through the camera 
station 5 and through the pictured point a (it intersects the ground 
plane in SoAo or in SqAi) we find from the similar triangles 



70 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

SSqA 1 and aaoA i the horizontal distance SqA i from the canaera 
station to the point sought, either graphically or arithmetically. 

Imagining the vertical plane SSqAi to be revolved about 
5o^i until it coincides with the ground plane GG, the points 5 
and a will assume the positions (S) and (a), Fig. 24, Plate XIII, 
and the line (5) (a) will pass through Ai, hence Ai may be 
located in the ground plane as the intersection of (5) (a) with 
Sodo- The same may be done for the other points Bi, Ci, and 
Z?i by revolving the vertical planes SSqBi, SSqCi, and SSqDi 
about Sobo, 5oCo,,and Sodo into the ground plane GG to locate 
the positions of Bi, C\, and Di. 

To avoid the drawing of so many auxiliary lines on the work- 
ing- or plotting-sheet, these constructions are preferably made 
on a separate sheet of paper, and the following method may be 
adopted : 

The vertical planes passing through SoO-oj Sobo, SoCq, and 
Sodo may be supposed to be revolved about SSo, as common 
axis of rotation, until they all coincide with the principal plane 
SSoPoj Fig. 25, Plate XIV, the surface of the paper representing 
the principal plane, when iTfl' = trace of the horizon plane 
in the principal plane, ilfiV = trace of the picture plane in 
the principal plane, GG = trace of the ground plane in the 
principal plane, 55o = /i= difference in elevation between the 
station 5 and the ground plane GG, measured in the plotting- 
scale, 5P = 50^0 = true length of the focal distance of the pho- 
tograph MN. 

The radials Soao, Sobo, SqCo, and Sodo are laid off upon the 
line GG from So=So{ao), Soibo), 5o(co), and So(do), and the 
verticals (ao)(fl)) (*o)(^), (co)(c), and {do)(d) are made equal to 
the ordinates aao, bbo, ccq, and ddo, respectively, measured on 
the picture. 

Radials drawn through (o), (6), (c), and {d) from S will 
cut off on the line GG the horizontal distances 5o(^), So{B), 
So(C), and So{D). These distances, laid off on the radials 
Sodo, Sobo, SqCo, and Sodo, on the plotting-sheet will locate, in 



TO PLOT A FIGURE BY MEANS OF ITS PERSPECTIVE. 7 1 

the scale of the map, the plotted positions of the characteristic 
points Ai, Bi, Ci, and Di of the lake, with reference to the ground 
line OqOq, which is identical on the plotting-sheet with the 
picture trace. 

We may reach the same results by utilizing the orthogonal 
projections of the points a, b, c, and d and those of Ai, Bi, Ci, 
and Di into the principal plane instead of revolving the ver- 
tical planes separately into the principal plane, as done above. 

With reference to Fig. 26, Plate XIV, we would then have:' 

FP= principal plane, MiV = picture plane, i?2T = horizon 
plane, containing the camera station S, GG = ground plane or 
surface plane of the lake A BCD. 

If we draw the radials SqOo, Sobo, SqCq, and Sodo from So 
(the orthogonal projection of 5 in GG) through the orthogonal 
projections of the pictured points a, b, c, d on the ground line 
OqOo', the points sought will fall upon those radials. After 
projecting the points a, b, c, and d, in the picture plane MN, 
upon the principal line ( = a, ^, y, and 8) the radials Sa, Sp, 
Sf, and Sd (drawn in the principal plane PP) will locate the 
points ao, /?9, ;-o, and ^oi respectively, upon the line SqPq (in 
the ground plane), and these represent the orthogonal projec- 
tions of the points A, B, C, and D in GG upon SqPo- Hence 
the points A, B, C, and D may be found by erecting perpen- 
diculars upon 50^0 in ctoi Po, To, and do, respectively, and their 
points of intersection with the radials Soflo •S'o&O) SqCq, and Sodo, 
respectively, will be the positions of the plotted points A, B, C, 
and D. 

Also this construction is preferably made upon a separate 
sheet of paper, Fig. 27, Plate XV, where the radials Sodo, Sobo, 
SoCo, and Sodo are drawn through their corresponding points 
on the plotted picture trace or ground line OqOo', but the rest 
of the construction is made on the separate sheet of paper, con- 
sidering the surface of the latter to' coincide with the principal 
plane (Fig. 28, Plate XV, where the designations are the same 
as in Fig. 25, Plate XIV). 



72 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The points 3, /?, a, and 7- on the line PPq (principal line) 
represent the projections into the principal plane of the pictured 
points a, b, c, and d, their positions being found by transferring 
the ordinates ddo, bbo, aag, and ccq of the pictured points d, b, 
a, and c to jPPq from Po, Po8=ddo, Po^=bbo, Poa = aao, and 

The radials from S through d, /?, a, and y locate the points 
^oi /^O) «0) S'lid xq on the line GG or on 5ojPo) and by transferring 
the distances So^o, SqPq, Soao, and SqYo, Fig. 28, Plate XV, to 
the principal line SqPq, Fig. 27, Plate XV, and drawing lines 
through ^oi /?0) ttoj and ;-o parallel with OqOq', their intersec- 
tions with the corresponding radials Sodo, Sobo, Soao, and SqCq 
will locate the plotted positions Dn Bi, Ai, and Ci of the points 
D, B, A, and C of the shore line of the lake. 



IX. To Draw the Horizontal Projection of a Plane Figure ABCD 
on the Ground Plan by Means of the So-called " Method of 
Squares," if its Perspective in Vertical Plane, abed, and the 
Elements of the Perspective are given. 

If we imagine the figure covered with a net of squares in 
such manner that one set of sides is parallel with, while the other 
is perpendicular to, the ground line, such net may be used to 
draw the outline of the figure upon the groimd plan. It will 
only remain necessary to cover the pictured figure abed with 
the perspective of the net that has been selected for the ground 
plan. The lines representing the squares in perspective must 
have the proper relation with reference to both, the principal 
ray and the horizon line, to conform with the net in the 
ground plan. 

The simplest disposition of the lines forming this auxiliary 
net is the one mentioned above, with one set of sides parallel 
with, and the other perpendicular to, the horizon line; still, any 
other disposition of the net lines or sides may be made: they 



THE "method of SQUARES." 73 

may form equal-sized squares or not and their directions may 
include any angle. 

In Fig. 29, Plate XVI, in illustration of this method, the 
lines of the perspective, corresponding to those sides of the 
rectangular figures that had been drawn at right angles to the 
ground line OqOo, will vanish in the principal point P, while 
those drawn parallel with the ground Hne OqOo' will be parallel 
with the horizon line 00'. 

Selecting the lines of this rectangular system so that one 
line of each system passes through each one of the characteristic 
points a, b, c, and d of the pictured lake, the perspective of this 
net will appear as shown by the fine lines in Fig. 29, Plate XVI, 
where OoOo' represents the ground line of the picture plane 
MN. 

If we again plot in the principal plane SSoPoPi Fig. 30, 
Plate XVI, and retain the same designations as in Fig. 25, Plate 
XIV, the points do, Po, ao, and yo will represent, in the ground 
plane GG, the intersections of the horizontal projection of the 
principal ray SP=SoPo with those net lines that had been drawn 
parallel with the ground line through D, B, A, and C. 

After plotting the picture trace OqOq, of the perspective 
MN, Fig. 29, Plate XVI, in the ground plan by means of the 
radials SoBq, Sobo, etc., Fig. 31, Plate XVII, the distances 5o^oi 
SqPo, etc., taken from Fig. 30, Plate XVI, and laid off upon 
SqPo, Fig. 31, Plate XVII, will locate the intersections of SqPq 
with those net lines (parallel with OqOo') in the ground plan 
that correspond to the lines dd, &/?, etc., of the perspective MN, 
Fig. 29, Plate XVI. 

If we now transfer the points ao', Po, bo', do', and Co' from. 
Fig. 29, Plate XVI, to the edge of a paper strip and place the 
latter upon the picture trace OqOo', Fig. 31, Plate XVII, that 
the points Po of both will coincide, then the lines ao'Ai, bo'Bi, 
etc., drawn parallel with SoPo will represent . those net lines 
that are perpendicular to the groimd line OqOq', and the plotted 
positions Au Bi, Ci, and X>i of the points A, B, C, and D are 



74 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

treated on the ground plan as the intersections of corresponding 
net lines of both systems as indicated in Fig. 31, Plate XVII. 

The points Ai, Bi, d; and Di will, of course, also be bisected 
by the radials Soaa, Sobo, SqCo; and Sodo, which fact may make 
some other disposition of the net lines more desirable for a figure 
of a different shape. 

When the figure is bounded by a sinuous perimeter, the 
squares of the net should be selected sufficiently small to enable 
the draughtsman to draw the perimeter sections falling within 
each square sufficiently accurate to obtain a correct reduction 
representing the general course of the figure's outline. 



X. The " Vanishing Scale." 

We had seen — Fig. 31, Plate XVII — that the radials drawn 
from the so-called foot Sq of the station 5 represent directions 
to the points Ax, Bi, Ci, and Z>i in the ground plane. If we 
now could determine from the perspective the distances (SqAi, 
SqBu etc.) from the foot So of the station to the points to be 
plotted their location in the ground plane would become an easy 
matter. 

The distances SqAi, SqBi, etc., may be determined from 
the perspective by means of the so-called vanishing scale, which 
may be constructed as follows, with reference to Fig. 32, 
Plate XVII, where ilfiV = trace of picture plane, ilil = trace 
of horizon plane, and GG = trace of ground plane, all in the 
principal plane, and where 55o = elevation of the station 5 
above the ground plane GG, or above the foot So of the station. 

A scale of equal parts is laid off upon GG to both sides of Pq 
and radials are then drawn from 5 through the graduation- 
marks. Their intersections with MN form the so-called van- 
ishing scale which may serve to locate the distances from the 
foot So of the station 5 to points that are to be plotted in the 
ground plane from the picture. 



THE "vanishing SCALE." 75 

The picture trace OqOo, Fig. 33, Plate XVIII, may have 
been plotted and the radials Soao, Sohj etc., may have been 
drawn on the working-sheet. It is desired to locate the posi- 
tion ^1 of a point A in the ground plane that is pictured as a 
in MN, Fig. 34, Plate XVIII, by means of the vanishing scale. 

Take the ordinate aao from the photographic perspective MN 
(the vertical distance of a above the ground line OqOo') and 
lay it off upon the vanishing scale PqP, Fig. 32, Plate XVII, 
from Po, equal to PqX. 

The line ax in the picture plane MN, Fig. 34, Plate XVIII, 
drawn parallel with the horizon line 00' and passing through a, 
is the perspective of the line AiX, drawn parallel with the ground 
line and passing through Ai, Fig. 33, Plate XVIII, in the ground 
plane. Hence, if we lay off ^o-X', Fig. 32, Plate XVII, upon 
SoPo, from 5o, Fig. 33, Plate XVIII, the point ^1 in the ground 
plane will be situated upon the line XAi, drawn parallel with 
the ground line OqOq' through X. The plotted position Ai 
of the point A will be at the intersection of the radial Soao with 
this line XAi. 



CHAPTER V. 
PHOTOGRAPHS ON INCLINED PLATES. 

Until now we have regarded phototopographic plates exposed 
in vertical planes, and although the general use of inclined, 
plates is not recommended for phototopographic purposes on 
account of the complications that will arise in the generally 
simple constructions underlying the iconometric plotting from 
vertically exposed plates, and because the relations that exist 
between the elements of the perspective and the orthogonal 
projection into horizontal plan will not be so readily recognized. 
Occasions may arise, however, where the selection of the availa- 
ble or accessible stations will be so circumscribed as to make 
exposures on inclined plates a necessity (to insure a good con- 
trol of the inaccessible terrene forms). Photographs may also 
have been obtained from balloons or with an ordinary camera 
not supplied with devices for adjusting the plate into vertical 
plane, or photographs originally taken for illustrative purposes 
may perchance find use for iconometric plotting. 

With reference to Fig. 35, Plate XIX, we have PP= prin- 
cipal plane, ^1?= horizontal plane passing through the nodal 
point of the camera-lens at station S, GG= ground plane, MN = 
picture plane, 0'P= trace of the picture plane MN in the hori- 
zon plane HH, Oo'Po= ground line of the picture plane, 5o=foot 
of the station S, P'Po= principal line of the picture plane, 
P'= principal point of the perspective MN, 55o= vertical of 
the station, S. It pierces the ground plane in the foot of the 
station and passes through the picture plane MN above (or 
below) the horizon line at s. The point s is the vanishing point 

76 



PHOTOGRAPHS ON INCLINED PLATES. 77 

for the perspectives of all vertical lines that may be pictured 
in MN. P'SP = P'sS=a= angle of inclination of the plate MN, 
5P= perpendicular through S to the horizon line O'P, 54= line 
of direction from 5 to a point A, pictured as a in MN. 

If -we revolve SP in the vertical plane PP about P until 
SP falls within the picture plane, the point 5 will fall into (5) , 
and the line 5a will fall into (S)a. 

The vertical plane, passing through 55o and containing 
the line 5^4, will intersect the ground plane in SoOq- If we 
revolve the line SqPq within the vertical plane PP about Pq 
until SoPo falls into the picture plane MN, the point So will 
fall into (5o) and the trace 5oOo will have assumed the position 
(5o)ao. 

The intersection ^o of the trace 5offo with the line of direc- 
tion Sa would locate the plotted position in GG of the pictured 
point a. 

The line sa intersects the ground line in ao, and 5oflo will 
be the radial in the ground plane from the foot So of the station 5 
that passes through the plotted position (in GG) of Aq. To 
find Ao on SqIo we first locate in the picture plane the inter- 
section (A) of the revolved lines {S)a and (So)ao- This point 
(A), revolved within the vertical plane ao5o5, will locate Aq 
upon Soao. 

To locate the position of 4 o in GG in the manner just indi- 
cated we should know the position of the line O'P, as well as 
the points 5 and P. These are known, or may readily be found, 
if the position of the principal point P', the length of the dis- 
tance line SP', and the value of the angle of inclination a are 
known. 

When a photographic plate is purposely exposed in an. inclined 
position in a. surveying camera, it will generally be done in such 
manner that the principal line //' still coincides with the inter- 
section of the picture plane MN and the principal plane PP, 
Fig. 35. Plate XIX. 

When the angle of inclination a is an angle of elevation (de^res- 



78 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

sion) the horizon line O'P will fall below (above) the line repre- 
senting the horizon line of the plate when exposed vertically. 

The angles of inclination for inclined plates should be 
observed directly in the field, and if the constant focal length 
of the camera =/ is known, the line SP may be found as the 
hypothenuse of the right-angle triangle having the angle = a 
and adjoining side = / = 5P'. 

A. To Plot the Picture Trace of an Inclined Plate. 

To plot the picture trace the horizontal angle included between 
\he optical axis of the inclined camera and the horizontal direc- 
tion to some known point should be known or measured. 

Should the length S'Si', Fig. 36, Plate XX, and the posi- 
tion of the line connecting two camera stations be known and 
also the position of a third point A, visible from both stations, 
no instrumental measurement of a horizontal angle a need 
be made, provided the plates containing the pictures a of the 
third point A are oriented in such manner that the picture a 
of that third point be bisected by the vertical thread, by the 
principal line //' of the perspective. 

We have with reference to Fig. 36, Plate XX: 5'= plotted 
position of the station 5, 5'5i'= plotted length and direction of 
the base line, S5 = elevation of the station 5 (laid off in the reduced 
plotting-scale). Fig. 37, Plate XXI. The horizontal angle a 
(at S', Fig. 36, Plate XX), included between the plotted base 
line S'Si' and the principal plane (or the horizontal projection 
S'Pq of the optical axis SP') may have been observed in the field. 

We revolve the line S'S about S'Pq, Fig. 36, Plate XX and 
Fig. 37, Plate XXI, into the plotting-plane, when it will assume 
the position S'{S), and erect at (5) a line (S)(P) perpendicular 
to S'{S). The angle of inclination of the plate MN = y is laid 
off from (5) upon (5)(P). We make (S)(P') equal to the 
constant focal length of the camera = /, when the line (/)(/')> 
drawn perpendicular to (S)(P') through (P), will represent 



PHOTOGRAPHS ON INCLINED PLATES. 79 

the principal line //' of the perspective MN, Fig. 37, Plate XXI, 
revolved about S'Pq into the plotting-plane. 

The point of intersection {s) of {S)S' with (/)(/') represents 
the vanishing point for all vertical lines that may be shown in 
picture MN. 

The intersection Pq of the perpendicular line (/)(/') with 
the horizontal projection of the optical axis S'Pq will be the 
trace of the incHned principal line //' in the ground plane (draw- 
ing plan). The line Pog, perpendicular in Pq to S'Pq, is the 
ground line or the trace of the inclined picture plane MN in 
the drawing plan GG. 

B. Plotting the Lines of Direction to Points pictured on an 
Inclined Photographic Plate. 

The inclined picture plane MN is revolved about Pog into 
the drawing or ground plane, Fig. 37, Plate XXI, when it will 
appear as (M){N), the principal point P falling upon 5'Po = 
(/)(/') in (P) and (P)Po is equal to PPq. 

To plot the direction from 5' to a point A, Fig. 36, Plate XX, 
pictured in MN as a, we first locate the orthogonal projection % 
of the pictured point a in the ground plane (plotting-plane). 
We project the image point a, Fig. 37, Plate XXI, upon //' or 
uponPPo=a, and describe a circle about Pq with Poa = Po(a)' 
to locate the position (a) of the projected point on the principal 
line (/)(/')) revolved into the ground plane. (The positions - 
of the pictured points a in Figs. 36 and 37 do not correspond;, 
both should be on the same side of //' in the picture planes» 
(M)iN)-.) 

The perpendicular to S'Pq, Fig. 37, Plate XXI, in ao and the 
vertical that passes through a intersect each other in ao- The 
point ao, Fig. 36, Plate XX, is located on the plotting-sheet as the 
intersection of (ao)«o (perpendicular to S'Pq through (ao) — ) 
and (a)ao (parallel with 5'Po or with (/)(/') through (a) -). 

S'ao, Fig. 36, Plate XX, will be the horizontal projection 



So PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

{in the plan) of the line of direction (or radial) from S' to the 
point A to be plotted. 



C. Determination of the Altitudes of Points pictured on an 
Inclined Plate. 

It is desired to find the elevation H of the point A, pictured 
in MN as a, above the ground plane GG. With reference to 
Fig. 36, Plate XX, the elevation aao=aao, in Fig. 37, Plate XXI, 
corresponds to (a)ao. 

If £)= horizontal distance of the plotted point A from the 
stations' (taken from the plotting-sheet), h=aao = aao = {a)ao, 
fl" = elevation of A above GG, and d=S'ao (Fig. 36), taken from 
the plotting-sheet, then the elevation H of the point A may be 
found, either graphically from a diagram. Fig. 39, Plate XXIII, 
«r it may be computed from the relation 

a 



D. Applications of Prof. Guido Hauck's Method. 

The constructions described for locating the horizontal direc- 
tions to points photographed on inclined plates may be greatly 
simplified by applying Prof. Hauck's method, utilizing the prop- 
erties of the " kernel points" of two photographs obtained from 
■different stations, but covering the same terrene. 

In Fig. 38, Plate XXII, 5 and S' may represent the tvsfo camera 
:stations. So and So' are the foot points of S and S' respectively, 
MN and M'N' may represent the inclined picture planes, both 
containing the images a and a', respectively, of a point A and the 
pictures 5' and s of the stations S' and S. The orthogonal pro- 
jections of the pictured points a and a' in the ground plane are 
ao and ao'- ^0 is the orthogonal projection of A into the ground 
plane GG. We had seen that I, s', and ;: are "kernel points " 



PHOTOGRAPHS ON INCLINED PLATES. 8 1 

for the picture plane MN and I', s, and n' are the "kernel 
points " for M'N'. 

The line connecting a and s' in MN and the line a's in M'N' 
intersect each other in the same point Q of the line of inter- 
section of the two pictures planes {MN and M'N'), and they 
also intersect the ground lines gg' in n and ;:' respectively. 

All lines in MN connecting s' with pictured points and those 
in M'N' connecting 5 with the images in M'N' of the same points 
will intersect each other in points Q of the line of intersection 
("perspective axis") of the picture planes. 

The points I and I' (the intersections of the verticals passing 
through the camera stations 5 and S' with the inclined picture- 
planes MN and M'N') are the vanishing points for the pictures, 
of all verticals shown in the negatives. Whenever the pictures^ 
contain images of vertical lines, the intersections of their pictures, 
would locate I and I' on MN and M'N' respectively ; still, when 
the picture plane is inclined in such a way that the principal 
line of the same would coincide with that of the vertically ex- 
posed plate (if the former were revolved about a line as axis 
passing through the second nodal point and being parallel with 
the horizon line 00', or HH'), the kernel point I may more 
readily be located upon //', as previously shown for 5 in Fig. 37, 
Plate XXI. 

The horizontal direction SqAq {Sq'Aq) intersects the ground 
line gg' of MN {M'N') in Oq (respectively in Oq'). Fig. 38, Plate 
XXIL In order to locate the position of .^o with reference 
to a on MN (to a' on M'N') we connect a and I (also a' with 
I'), which line locates Aq (and ao') upon the ground line gg' of 
the picture plane MN (and M'N' respectively). 

The interesction Aq oi the lines Soao and S'ao' will now give 
the plotted position in the ground plane GG of the point A. 



CHAPTER VI. 

PHOTOTOPOGRAPHIC SURVEYING METHODS. 

From the preceding chapters we find that in order to utilize 

for iconometric purposes the data contained in a photographic 

perspective we should know: 

First. The three constants or elements of the perspective, 

which are the focal length, together with the horizon and 

the principal Une, or the focal length and the principal 

point, together with either the horizon line or the principal 

line of the perspective. 

Second. The position of the picture plane with reference to 

fixed points of the terrene, which means the elements for 

the orientation of the picture trace in the plotting-plane. 

To plot the position of any geodetic point in both the horizontal 

and in the vertical sense, we should know, or be able to ascertain, 

First. The horizontal angles included between the principal 

plane and the lines of direction from two or more stations 

to the geodetic point. 

Second. The angle of elevation (or depression) which is the 

vertical angle included between the horizon plane and the 

line of direction to the geodetic point. 

If the constants or the elements of the perspective are known, 

the geodetic elements (the horizontal and vertical angles) needed 

for plotting the position of the geodetic point may be ascertained 

either graphically or arithmetically. 

Phototopographic methods being generally applied with a view 

toward obtaining a graphic record of the measurements in the 

82 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 83 

form of cartographic representation of the terrene, we shall give 
in these pages principally graphic solutions of the more important 
problems met with in phototopography. 

I. Analytical or Arithmetical Phototopographic Methods. 

A. Method of Prof. Jordan. 

In Chapter I, section III, mention has been made of Prof. 
Jordan's map of the oasis "Dachel " and village "Gassr-Dachel," 
based on Remele's photographs. Care was exercised to expose 
the plates in vertical plane, and horizontal directions to at least 
three points of each photograph were measured instrumentally 
to obtain the required data for the orientation of the pictures. 
Vertical angles to at least two such points for every picture were 
also observed to give the means for locating the horizon lines 
of the pictures, thus enabling the draughtsman to deduce the 
elevations of other points pictured on the photographs. With 
reference to Fig. 40, Plate XXIII, we have : 

00'= horizon line of photographic perspective MN; 
//'= principal line; 
P= principal point; 

5 = second nodal point (focus) of camera lens; 
5i' = / = focal length of picture ikf-A/' = principal ray; 
a, b, and c = images of three points A, B, and C; 
«!, a2, and aa^horizontal angles a'SP, b'SP, and c'SP; 

5iV= direction of the meridian passing through the 
station 5; 
^1 ^2) and ^3 = azimuthal angles NSa', NSb', and NSc' re- 
spectively; 
Hi, H2, and i?3 = elevations of the points A, B, and C above the 
plane of reference or ground' plane. 

The photographic plate MN having been exposed in vertical 
plane, it will be evident that for the three points a, b, and c 
respectively the abscissae xi, X2, and X3 should be 



84 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Xi—f tan ai, 
X2 = j tan ag, 
X3=} tanas, 

,,, ; . sin(a2-ai) 

or iC2— ^i=/(tana2 — tanai)=/-- 



and ^3— a;2 = /(tana3 — tana2)=/ 



cos ai cos a2 

sin (a3-a2) 
cos as cos a2' 



The values for (a;2— xi) and (ais— ;x;2) may be scaled off directly 
on the negative, MN, and the values for (a2— ai) and (aa— a2) 
may be taken from the field records of the observed horizontal 

angles, when the value for may be computed by means 

of l;he formula 

^2— ^i_cosa3 sin (a2— ai) 
^s — ^2 cosai sin (as— a2)' 

If we substitute tan r for -, and as ' 

cos ax 

I +tan Y 

^3^^ = tan(45+r), 



we may write 




, cos as 

, . , . cosai 

tan (45 +r)= = 

^^•^ ' ^ cos as 


cosai+cosas 
cos ai - cos as 


cosai 




_ ai+a3__ ai-as 





. ai+as . ai-as 
sm sm • 



.co.(2l±a)eo,(!az2.), 



hence tan SiS-cot (4S + r) cot (^^). 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 85 

From this equation ai+as may be computed. 
By inspection we find from Fig. 40, Plate XXIII, 

a2-«i = ^2-'^i = £i. 
By adding these two equations we obtain 

«1 -"3 = 02-^3- 

Knowing (ai +0:3) and (ai —0:3) we can readily find cci and 0:3; 
also a2=ai + £i 

or =a3 — £2- 

We had found 

, sin (a2— «i) I sin ii 
X2-Xi = f =/- 



hence '= 



cos ai cos aa cos a\ cos a2 
{X2—Xx) COS «! cos a2 



sm £1 



sin (as -0:2) , sin £2 
and «^3-^2=;— —-——- = / 



cos as cos a2 cos as cos a2 
(xa — X2) cos ciz cos a2 



whence /= 



Thus two elements of the perspective MN, the focal length /, 
and the principal line //' (given by the abscissas xx, X2, and x^), 
may be found. 

With the aid of the observed vertical angles /?, the third ele- 
ment, the horizon Ime 00', may now be located on the photo- 
graph. 



86 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The vertical angle ^3=cSc' having been observed at S to 
the point C we find 

cc' = ya = Sc' ■ tan 83 = tan 83, 

■' ^ cos as 

and for the point a the vertical distance to the horizon line would 
be 

aoi = yi = Sa' • tan A = tan 81, 

■' '^ cosai "^ 

The horizon line 00' will be the common tangent to two 

circles, one described with the radius = ■ tan /?i about a 

cos a\ 

and the other with a radius = tan 83 about c. 

cos as 

At least two vertical angles having been observed for each 

exposed plate, the horizon line 00' may thus be located and 

marked upon the negative, when the principal point P may 

also be located on 00' by means of the principal line //', the 

latter being tangent to the three circles described about a, 6, 

and c with the radii X\, X2, and X3 respectively. 



B. Method of Dr. Le Bon. 

Dr. G. Le Bon (who used his instrument chiefly for the plot- 
ting of ancient buildings and monuments in India) provided 
the ground-glass plate of his camera with a net of squares, each 
square having i cm. sides, one set of the latter being drawn 
parallel with the horizon, while the second set of lines is paral- 
lel with the principal line of the perspective. The lines repre- 
senting the horizon and principal hnes are again subdivided into 
millimeters. 

This arrangement enables the operator to obtain the measure- 
ments of objects directly by inspection of the image on the gradu- 
ated ground-glass plate. 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 87 

To determine the dimensions of the front of a building Dr. Le 
Bon measures a certain length directly upon the same and then 
takes a picture by exposing a photographic plate in vertical 
plane and parallel with the base of the front (facade) of the 
building. 

For example, to find 

First. The distance D of an object, the height H of which 
is not known. Fig. 41, Plate XXIII: 

Two stations 5 and S' are occupied on a base line B (which 
is measured directly in the field) laid off in a direction perpen- 
dicular to the base of the object. 

If the height of the image measured on the ground glass 
at the first station is h, at the second station h', and if the focal 
length for both exposures be the same and=/, then 

and for the second station S' 

(D+B):H=}:h'. 

h and h' being known — they may be measured directly on the 
negative or on the groimd-glass plate — ^we find, after dividing 
the second equation by the first, 

D+B h 
D "h" 

B h h-¥ 



^^ D~h' ^ h' ' 

Bh' 



whence D — 



h-h'' 



88 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Second. The height H of an object is to be found when 

the fractional length H' has been obtained by direct 

measurement (Fig. 42, Plate XXIV). 

On the image of the object — on the graduated ground-glass 

plate — the lengths for the heights h and h' may be read off directly, 

and as H' is also known, we find H from the equation 



C. Method of L. P. Paganini {Italian Method). 

This method has been extensively used for the new topo- 
graphic survey of the kingdom of Italy and for the Colonial 
possessions in East Africa (" Eritrea"). 



COMPUTATION OF THE ELEMENTS OF THE PHOTOGRAPHIC 

PERSPECTIVE WHICH ARE NEEDED FOR THE 

ICONOMETRIC PLOTTING. 

I. Determination as the Focal Length of the Photographic 
Perspective. 

(a) When the Reference Point is Bisected by the Principal Line of the 

Perspective. 

A triangulation point 5, Fig. 121, Plate LX, may be visible 
from the camera station V. The camera is directed toward 5 
in such manner that the image s of the distant peak S is bisected 
by the vertical thread //'. 

F= camera station or the point of view of the perspective 

MN; 
Pi = principal point of the photograph; 
VPi = focal length or distance line of the perspective, denoted 
by/; 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 89 

5' = orthogonal projection of S in the horizontal plane which 
passes through V; 
F5i' = horizontal distance from V to S, designated by D; 
55' = apparent difference in elevation between V and 5, des- 
ignated by L. 
After having carefully measured the ordinate Ps = y on the 
negative, we can determine the focal length from the equation 

' L • 

Example No. I. — ^The station V may be occupied over the 
centre of the triangulation point Reale Accampamento and the 
bisected point 5 be \he signal upon Cian del Lei. The camera 
having been leveled and adjusted over Reale Accampamento is 
turned in azimuth until the signal Cian del Lei is bisected by the 
vertical thread //' and the first plate is then exposed (Fig. 122, 
Plate LXI). 

The focal distance, read off on the scale a of the lens tube 
= 244.5 mm., and the values fori) and L, taken from the records 
of the new trigonometrical survey of Italy, are 

i?= distance from Reale Accampamento (signal) to 

Punta Cian del Lei (signal) =3270.7 m. 

Elevation of station mark at Punta Cian del Lei =2811.72 m. 
Elevation of camera horizon at Reale Accampamento = 2 191.80 m. 



Difference of true elevation = 619.92 m. 

The ordinate y, carefully measured on the negative (from the 
principal point P to the image s of the point Cian del Lei) gave 
46.25 mm. 

Computation of L : 

True difference in elevation =619.92 m. 

Correction for curvature and refraction = —0.72 m. 



Z, = apparent difference in elevation = 61 9. 20 m. 



90 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Computation of /: 

logD= log 3270.7 =3.5146407 

log y= log 0.04625=8.6651117 

colog Z, = colog 619.20 =7.2081691 



log /= 9.38792 1 5 

/ = 244-30 
Scale-reading for / was = 244. 50 



Difference = 0.20 mm. 



(b) The Image of the Reference Point Falls to either Side of the Principal 
Line of the Photographic Perspective. 

If the image s of the reference point 5 be to either side of 
the vertical thread //' of the perspective MN, Fig; 123, Plate 
LX, the principal ray VP making an angle e with the horizontal 
direction VS' to the reference point s, then the value VP=f may 
be found as follows : 

<f= horizontal distance Vs' (Fig. 123); 

y and a; = coordinates of the image s; 

£)= horizontal distance between the camera station V and the 

reference point S, and 
Z,= apparent difference in elevation between V (or s') and s. 

From the similar triangles VSS' and Vss' we find 
D d' 



:, Dy 
hence a = -y— . 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 9 1 

From the horizontal triangle s'PV we find 



^=^ 



cos e 



hence /=— ^cos e. 

Example No. II. — In the panorama (series of ten photographic 
perspectives) obtained September 21, 1884, vertically above the 
trigonometrical point (of the new Italian geodetic triangulation 
system), near Reale Accampamento of Valsavaranche, there is 
one plate (P^, Fig. 122, Plate LXI) which contains the image 
of the triangulation station Punta Ruja (signal). 

The horizontal angle w, between the optical axis of the camera 
for this plate and the horizontal direction to Ruja (signal), is = 

5° 49' 27".7S- 

The horizontal distance: Reale Accampamento — Ruja = Z' = 
5804.2 m. and the elevation of Ruja = 31 73.5 m. are taken from 
the triangulation data. 

By careful measurement y is found to be =41.45 mm. 

It is desired to find the focal length, /, for this perspective, 
which may be obtained, approximately, by reading the gradua- 
tion on the objective tube = 244.50 mm. 

Computation of L : 

Elevation of station mark at Ruja =3173.5 m. 
Elevation of the camera horizon (00') = 2i9i.8 m. 



True difference in elevation = 981.7 m. 
Correction for refraction and curvature = — 2.3 m. 



Apparent difference in elevation = 979.4=!, 



92 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Computation of /: 

log £» = log 5804.2 m. =3.7637424 

log >' = log 0.04145 m. =8.6175245 

log cos oj = log cos 5° 49' 27".75 =9.9977522 

colog i= colog 979,4 = 7.0090399 



log/ =2.3880590 
/ = 244.38 mm. 
Scale reading = 244.50 mm. 



Difference = 0.12 mm. 

Had we measured the abscissa x instead .of the ordinate y 
the focal length for the negative could have been computed by 
the formula 

f=x cot 0). 

Example No. III. — ^The measured value for x may have been 
found to be =24.90 mm. 
Computation of /: 

log ;x;=log 24.90 =1.3961993 

log cot a. = log cot 5° 49' 27".75 =0.9913737 



log / = 2.3875730 
/= 244.103 mm. 



2. Orientation of the Picture Traces. 

There exists a close connection between the phototopographic 
stations and the new triangulation of Italy. A generous dis- 
position of the trigonometric points had been made with the 
special purpose in view that they were to serve as the foundation 
for the subsequent topographical survey. These points have 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 93 

been carefully selected, their positions have been precisely com- 
puted, and their locations have been permanently marked in the 
field, irrespective of the character of the surrounding topography 
or of the order of triangulation to which the point may belong. 
This large number of triangulation points not only facilitates 
the application of the phototopographic surveying method and 
assures the accurate determination of the panorama stations 
(in the horizontal and vertical sense), but it also greatly simplifies 
the subsequent iconometric plotting, as the greater part of the 
perspective contains one, two, or more pictured triangulation 
points, notwithstanding the instrument commands a field of view 
of but 42° horizontally. Two adjoining plates have a common 
margin of an angular width of 3°, reducing the effective field of 
view of one plate to 36° (Fig. 124, Plate LXII). 

Thus the picture traces are easily oriented for the icono- 
metric work, the salient topographic features (deduced from 
the perspectives) may be frequently checked, and such nega- 
tives (containing the images of triangulation points) may also 
serve to verify the focal length / of the panorama pictures, check 
the position of the principal point P, and they give the means 
for testing the location of the horizon line 00' on the pictures. 

The perspective MN, Fig. 125, Plate LXII, may contain 
the images of two trigonometrical points S and S'. 

In the preceding pages it has been shown that the horizontal 
distances, d and d', from the camera station V to the pictured 
points S and S' may be found from the relations 

Dy , D'-y 

d=—jr- and d' = — jT' ■ 

In the triangle VSiSi' we know the lengths of two sides, d and 
d', and also the value of the included angle, SiVSi', which may 
be either measured directly at the camera station or, when the 
latter is also a triangulation station, the value for the angle may 
be taken from the triangulation records. 



94 PHOTOTOPOGRAPHIC METHODS AND INSTRtJMENTS. 

The other two angles, y and d, of this triangle may now be 
found as follows: 

r-8 d'-d V 
tan = „ , , cot - . 

2 fl'+O 2 ' 

r+8 V 

^+54.7=180°, hence =90° — . 

If we replace W+S) by M and Ut-^) by N, we find after 
adding the equations 

and by subtracting the equations 

ii[r+d-r + 8]=M-N=d. 

The principal ray VP should be vertical to the horizon line 00^ 
and both triangles VPSi and VPSi' should be right-angle triangles. 
Hence the focal length / should be 

/=<^-sin ]r 
and f = d' -sin d. 

To ascertain whether the pictured intersection of the cross- 
wires P coincides with the principal point of view P upon the 
perspective, the measured lengths of the abscissae x and x' 
(Fig. 125, Plate LXII) should be the same as the computed 
values 

x = f cot ;-, 
x'=jcotd. 



The angles of orientation w and to' are: 

w=9o°- 
a/ = go°- 



oj = go°-r, 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 95 



3. Determination of the Elevations or Pictured Terrene Points. 

The vertical angles of elevation a and a' of the two refer- 
ence points 5 and S' may be computed from the equations: 

L 
tana =— , 

, U 
tana'=-^. 

These angles are either taken from the triangulation records 
or they may be observed directly from the camera station, and 
to check the position of the horizon line 00' the ordinates 
y and y', measured on the perspective, are compared with those 
computed by means of the equations 

y= tana, 

•^ cos (D 

y = 7 tana'. 

•^ cos U)' 

Example No. IV. — In the panorama obtained Sept. 19, 1884, 
from Punta Percia (this peak is on the divide separating the 
valleys of the Rhemes and the Valsavaranche) two trigonomet- 
rical stations, Pvmta Rouletta and Gran Punta di Nomenon, of 
the new Italian geodetic survey appear upon the same plate (see 
Fig. 126, Plate LXIII). 

The following values are given for the computation: 

Elevation of Punta Rouletta =3384.10 m. f Taken from the 

•j catalogue of tri- 
Elevation of Punta di Nomenon = 3488.42 m. [ angulation points 

f Elevation of P . 
Elevationof horizon of camera station (P. Percia) = 3202.3 m. ■! Percia -I- height 

[ of instrument 

f Measured upon 
Distance: Percia-Rouletta=D =3250 m. the projection of 

■ the iconometric 
Distance: Percia-Nomenon= D' =9720 m. working sheet, 

[ scale Kxr^trtr 



96 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The horizontal angle V (Fig. 126, Plate LXIII) at Punta 
Percia, included by the horizontal directions to Rouletta (signal) 
and Nomenon (signal), =28° 02' 30". 

A careful measurement of the coordinates of the pictured 
points P. Rouletta and P. Nomenon, on the negative with 
a millimeter scale provided with a microscope and vernier, en- 
abling the computer to read to 0.05 mm. (the vernier is divided 
to read to 1/20 of the graduation unit), produced the following 
values : 

The coordinates of Punta Rouletta, 

x=46.o5 mm.; 3' = i3.75mm. 
The coordinates of Punta di Nomenon, 

jc' = 75.40 mm.; / = 7.30 mm. 

It is desired to find: 

(i) The focal distance for this negative = /, the preliminary 
value, read off on the scale attached to the objective cylinder, is 
found to be 244.50 mm. 

(2) The correct position of the principal point (P), which 
will be fixed by the determination of the abscissae x and oc'. 

(3) The position of the line of horizon 00', which will be 
located by ascertaining the values for y and y'. 

Computation to determine the apparent differences in elevation 
between the camera horizon (P. Percia) and the two pictured points 
P. Rouletta and Punta di Nomenon. 

Altitude of P. Rouletta =3384.10 m. 

Altitude of camera horizon = 3202 . 30 m. 



True difference in elevation = 181 .80 m. 
Correction for curvature and refraction = — 0.71 m. 



Apparent difference in elevation =Z, = 181 .09 m. 
Altitude of Punta di Nomenon =3488.42 m. 

Altitude of camera horizon =3202 .30 m. 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 97 

True difference in elevation = 286 . 1 2 m. 
Correction for curvature and refraction = — 6.35 m. 

Apparent difference in elevation —L'— 279.77 



Computation of (i= — y. 

log Z)=3.5ii8834 

logy =8.1383027 

cologI.= 7.742105s 

log (i= 9.3922916 
d= 246.77 mm. 



D' 
Computation of d'=—y', 

log r»' =3.9876663 

log y'= 7.8633229 

coIogL'= 7.5531989 

log (^'=9.4041881 
d'= 253.62 mm. 



d+(i'= 500.39 
d'-d= 6.85 

Computation of the angles j and 8: 



r-d d'-d V 

tan = ;j^-— j-cot— ; 

2 d' +d 2 

F = 28°o2'3o"; - = 14° 01' 15"; 



j-+a=i8o-F=i5i° 57' 30"; ^=75° 58' AS"=M. 

log (<i'—<i)= log 0.00685 =7.8356906 

V 
log cot - = logcoti4° 01' 15" =0.6025567 

colog (<i'+(^) = colog 0.50039 =0.3006914 



logtanL = 8_738g38y 

r-d 

'- =3° 08' i6".i=iV. 

2 

Hence M+N=r=79° of oi".i 

M-N = d='j2° 50° 28".9 



98 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

(a) Computation ef the Focal Length ( =/). 

/=(i-sin 7- . /=</'-sin5 

log d = 9. 3922916 log rf' =9.4041881 

log sin 7-=9.992ii8o log sin 5 = 9.9802269 



log / = 9 . 3844096 log / = 9 • 38441 SO 

/=o. 242331 m. /=o. 242334 m. 

Mean value for / = 242. 332 mm. 



{j8) Computation of the Abscissa (x and x') for Platting Lines of Hori- 
zontal Directions to Pictured Points of the Terrene and for Checking 
the Position of the Principal Point. 

x=/-tanw x'=j-ta.nw' 

CO = go°-r = 10° 52' S8".9 oj'==go°-d = if 09' 31".! 
log / = 9 . 38441 23 (mean log) log / = 9 . 38441 23 (mean log) 
log tan w = 9. 2838945 log tan ^' = 9. 4896222 



log x= 8. 668 3068 log j(/ = 8.8740345 

a;=46.59mm. 5(/ = 74.82 mm. 

X meas. on plate=46.o5 mm. x' meas. on plate = 75.4o mm. 
difference = Jx=o. 54 mm. Jaf =0.58 mm. 

Mean di£Eerence=o.56 mm. 

From this difference we infer that the principal point P oi 
the photographic perspective should be transposed toward the 
pictured point of Punta di Nomenon by 0.6 mm. 

(;-) Computation of the Ordinates (y and y') of Pictured Terrene Points 
t)f Known Elevations to Check the Position of the Horizon Line (,00') 
on the Negative. 

y = tan a, y' = ;tan a'. 

"^ cos a» ■' cos (J 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 99 

Where 



«= angle of elevation of Punta Rouletta =3° 11' 30" 
a'= angle of elevation of Punta di Nomenon= 1° 38' 30" 



' Observed from the trig- 
onometrical statioa 
Punta Percia 



log /= 9-38441 23 

log tan a= 8.7463444 

colog cos (0=0.0078820 



log 51=8.1386387 
y= 13.761 mm. 
51 measured onplate= 13.75 ™™- 

Difference= o.oi mm. 



log /= 9-3844123 

log tan q:'= 8.4572812 

colog cos &)'= 0.0197731 

log y'= 7.8614666 
y'= 7.269 mm. 
y' measured= 7.30 mm. 

Di£ference=o.03 mm. 



The correction for y is so small that it may be disregarded; 
the length measured on the plate for / should be reduced by 
0.03 mm. and the corrected horizon line would fall 7.27 mm. 
below the pictured point P. di Nomenon (Fig. 126, Plate LXIII). 

(3) Orienting a Panorama. 

The angles of orientation of the perspective (w and co') regard- 
ing the two pictured triangulation stations had been found to be 

a> = io° 52' s8".9 
and (o' = if 09' 3i".i 

Owing to the fact that the distance D and D' in the pre- 
ceding example are large in comparison with the ordiuates 
y and y', it may be preferable first to determine / by means of 
the abscissae and then to compute the values for the ordinates 
(y and /) based upon this value of / and the observed angles 
of orientation co and a/. 

If we construct the decagon (see Fig. 124, Plate LXII) rep- 
resenting the horizontal projection of the ten negatives obtained 
at the station Punta Percia by means of the elements, obtained 
by ofeervations only, we will find the direction to the principal 
point P of the perspective containing the pictures of Rouletta 
and Nomenon to be = 350° 00' 00". 



lOO PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Direction to Punta Nomenon (signal) = 332° 42' 00'. 
Direction to Punta Rouletta (signal) = 0° 44' 30". 
From these lines of direction we find the following values for 
the angles of orientation: 



Direction to Rouletta = 360° 44' 30" 
Direction to point P =350° 00' 00" 



iu= 10" 44' 30" 



Direction to point P — 350° 00' 00 ' 
Direction to Cima 

Nomenon =332° 42' 00" 



Which differ from <"'= 17° 18' 00" 

the preceding values <u= 10° 52' 58". 9 

by J<u= ± 8' 28".9 <i''= 17' 09' 31". I 

This small angle Jco (at V) corresponds to an abscissa Ax,. 
which in turn represents the error in position of the pictured 
point P in the horizontal sense. (In the preceding example, 
panorama from station Percia, Sept. 19, 1884, J:)(;=o.56 mm.) 

From the right-angle triangle with angle at F = Jw and the 
two sides Jx and / (Fig. 126, Plate LXIII) we find the value 
for Jx for our negative from the equation 

Jx = f tan Joj. 
log /=log 242.332 =2.3844123 
log tan 0° 08' 28".9 = 7. 3922081 



log J:v = 9. 7766204 
Jx= To. 598 mm., 

representing the error in the position of the principal point of 
the perspective, which, however, has little iniluence upon the 
precision of the graphical operations (iconometric plotting) 
which are to be executed in order to transpose the topographic 
relief upon the chart from the photographic perspectives as 
long as the error in question (Jx) does not exceed 2 mm. for 
the entire panorama of ten plates, each controlling an angle 
of 36° horizontally. 

4. Checking the Verticality of an Exposed Plate. 
For mountain work, where the differences in elevation between 
the several terrene points (which are pictured on the negative) 
and the camera horizon are relatively great, it is important to 



PHOTOTOPOGRAPHIC SURVEYING METHODS. lOI 

know whether the negative had been exposed while in vertical 
plan and whether the photographic perspective has a correct 
horizon line OO', since the ordinates of the various terrene 
points in question would become too long when the plate is 
(inclined) not vertical. Therefore to assure oneself whether 
the picture plane (ground-glass or negative plate) of the photo- 
theodolite is vertical and also whether the optical axis of the 
camera is horizontal (the trace of this axis upon the perspective 
plane is represented by the intersection P of the pictured cross- 
threads 00' and //') bring the plane containing the optical axis 
and the " horizontal wire " 00' representing the horizon line 
into horizontal plan and direct the instrument to a well-defined 
distant point situated high above the camera horizon, and 
at the same time the vertical thread should be visible against 
the ground glass, so that the distant point may be bisected by 
it. If we now measure the ordinate of the elevated distant 
point on the ground -glass plate and repeat this measurement 
after revolving the camera in aziinuth i8o°, clamping the hori- 
zontal circle, and " transiting " the camera (revolving it about 
its horizontal axis of revolution i8o°), the two lengths thus 
obtained for the ordinate of the point in question should be the 
same, otherwise the conditions of the apparatus are not satis- 
factory and the instrument should be adjusted until the two 
measurements give the same results. 

With reference to Fig. 127, Plate LXIII, we have 

FP = optical axis of the camera made horizontal and the vertical 
thread bisecting the distant point S; 
MP A = angle of inclination of the picture plane against the verti- 
cal plane; 
Po= length of ordinate of pictured point S, measured on ver- 
tical thread from P. 

The plane of the perspective NM not being vertical to the 
optical axis, it will assume the position N'M' for the indirect 
position of the camera (as described above) and the ordinate Po' 



102 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

measured on the ground-glass plate will now be shorter than 
when measured {=Po) before. Had the picture plane been 
vertical originally it would have coincided with AB for both the 
direct and reversed observation; the ordinate measured in both 
positions would have been = Ps. 

The instrument must be adjusted to make 



Po+Po' 
Ps = — . 



The error in position of the principal point P of the perspective, 
considered in the last numerical example, will appear immediately 
if one wishes to determine the value of the focal length =/ by 
means of the abscissae measured on the ground-glass plate (or on 
the negative). 

We had found by measurement 

re =46.05 mm. and a:' = 75.40 mm. 

and the observed correpsonding angles of orientation were 

aj = 10° 44' 30" .and oj' = i'j° 18' 00". 

The twofold determination of / may be derived from the 
relations 

/=^ and /=- 



tan 0) tan to' 



log «= 1. 6632296 
cologtan ft)= 0.7219207 



log /= 2. 385 1 503 
/= 242. 745 mm. 



log »'= 1. 8773713 
cologtan aj'= 0.5065903 

log /= 2. 3839616 
/= 242.082 mm. 



Mean value for /= 242.41 mm., differing but slightly from the value /= 242.33 
mm., previously obtained. Difference=o.o8 mm. ' 

Example No. V. — Giving the means for ascertaining the 
attainable degree of accuracy of the Italian phototopographic 



PHOTOTOPOGRAPHIC SURVEYING METHODS. I03 

method the following computation is of greater interest in a general 
way, as the panorama station was selected over a trigonometrical 
point of the Italian geodetic triangulation system, thus admitting 
a direct comparison between the elements of the perspective 
and the exact values of these same elements deduced from the 
data of the triangulation work. 

In the panorama views, obtained on Sept. 21, 1884 (see Ex- 
ample No. II), vertically above the trigonometrical point known 
as Reale Accampamento there is one plate (P^) which con- 
tains the pictures of two triangulation points, Pxmta Ruja 
and Gran Cima di Nomenon (the same points as previously 
mentioned). From the geodetic computations we take the follow- 
ing data: 

Elevation of Punta Ruja (signal-mark) =3i73'S m- 

Elevation of P. di Nomenon (signal-mark) =3488.4 m. 

Elevation of camera horizon (Reale Accampa- 
mento) = 2191.8 m. 

Distance: R. Accampamento-Ruja=£) =5804.2 m. 

Distance: R. Accampamento-Nomenon=Z)' =5029.6 m 

Horizontal angle: F=Ruja-Accan3.pamento- 

Nomenon =13° 51' 04". 50 

It is desired to find (see Fig. 128, Plate LXIV): 

(i) The focal length, /, approximately found by reading the 
scale attached to the objective tube = 244.50 mm. 

(2) The position of the principal point of view P which is 
located by the abscissae x and otf. 

The coordinates obtained by carefully executed measurements 
on the negative (the same as in the preceding) are for 

Pimta Ruja: x =24.80 mm.; y =41.45 mm. 

Gran P. di Nomenon: x' = 34.05 mm.; / = 63.50 mm. 



104 



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



Computation of the apparent difference in elevation: 



Elevation of P. Ruja =3173.5 m. 
Elevation of camera ho- 
rizon = 2191.8 m. 



True difference in level= 981.7 m. 
Correction for curvature 

and refraction — 2.3 



Apparent difference of 

elevation = 979.4 m.= L 

Computation of d— — -y. 

log 0=3.7637424 

logy=8. 6175245 

colog L= 7 . 0090399 



Elevation of Nomon = 3488.4 m. 
Elevation of camera ho- 
rizon = 2191.8 m. 



True difference in level= 1296.6 m. 
Correction for curvature 

and refraction — 1.7 m. 



log d= 9. 3903068 
d= 245 . 644 mm. 



Apparent difference of 
elevation =i294.9m.=L* 

D' 
Computation of d'=—,-y'. 

log D'= 3. 7015334 

log y'= 8. 8027737 

colog £'=6.8877638 



log d'—g. 3920709 
(2'=246.644mm. 



(i-Fd'= 492.29 mm. 
d'—d— i.oomm. 



Computation of the angles y and d: 



^-a d'-d ^ V 



7=i3<'5i'o4".5o; 



:=6°5S'32".2S. 



7-1-5= 180- F= 166° 08' 55 '.50 

I±i=M = 83° 04' 27".7S 

log (d'—d)= 7.0000000 
V 



log cot 



=0.9155404 



colog (d-|-d')=o-3077790 
log tan ^— 8.2233194 



r-8_ 



0° 57' 29".io=iV 



M+N=S4° 01' 56".9 =r 
M-N==%2° 06' 58".7 =d 
Computation of f; 

/= d sin J-. 
log d= 9. 3903068 
log sin ;-= 9 . 9976402 



log /= 9. 3879470 
/= 0.244313 m. 



/= d' sin 8. 
log d'= 9 . 3920709 
log sin 5=9.9958757 



log/ = 9 3879466 
/= 0.244313 m. 



PHOTOTOPOGRAPHIC SURVEYING METHODS. IO5 

Mean value for /= 244.31 mm. is the same as obtained in numerical example 
No. I. 

Computation of the abscissa x and x' {to check the position of the principal 
point P) : 



x=f tan u). 
w=90°— r=5° 58' 03".! 
log/= 9.3879468 
log tan <u= 9.0192462 



loga;= 8.4071930 

x=25.54 mm. 

Measured x= 24 . 80 mm. 



Di£f.= 0.74 mm. 



x'—f tan to'. 
a)'^go°-d=f S3' oi".3 

•og/= 9 3879468 
log tan <u'= 9.1413601 



\ogx'= 8.5293069 

a;'= 33.83 mm. 

Measured »'= 34 . 05 mm. 



Diff. = 0.22 mm. 



The mean difference=o.48 mm. = error in the position of the principal point P 
of the perspective P* (Accamparaento panorama). The true point P' is 0.5 mm. 
to the right of P^ Fig. 122, Plate LXI, and the vertical line //' (the principal line) 
on the plate, Fig. 128, Plate LXIV, should be moved towards Punta di Nomenon 
(image) 0.5 mm. 

The preceding five numerical examples may well serve to 
elucidate the relations between the elements of the photographic 
perspectives and their corresponding parts of the terrene, as well 
as to give the means for forming a correct idea of the degree 
of accuracy, attainable in the Italian photographic surveying 
method. 

In practical work it would be too time consuming to make 
such computations (with the necessary minute and careful graph- 
ical measurements) for every negative, or even for every set of 
panorama views. 

If the phototheodolite h^s been carefully planned and is well 
constructed, the optical axis should always remain perpendicular 
to the image plane, hence be horizontal when the latter is ver- 
tical. The value / for any (or for all) panorama views, obtained 
with the same objective and with the same constant focal length 
(obtained under the same reading of the scale attached to the 
objective tube), may be computed from the formula 



' tan 18°' 



I06 PHOTOTOPOGEAPHIC ■ METHODS AND INSTRUMENTS. 

In the panorama set of Accampamento Reale station we 
had one plate, P', Fig. 122, Plate LXI, containing the image of 
the signal at Punta Cian del Lei, bisected by the principal 
line, VP', and another plate, P^, containing the images of two 
other points, Punta Nomenon and Punta Ruja. 

The horizontal shiftings in azimuth 

P'VP^, PWP^ PWP*..., 

representing the horizontal swings in azimuth of the camera for 
each successive exposure, are all alike, each being =36°, and the 
horizontal angles 

P'Vm, mVP^, PWm' . . . 

will each be =18°. 

The horizontal angle, included between the principal lines 
VP' and VP^ (between the horizontal direction to the reference 
point P. Cian del Lei and the principal line of perspective 
No. s) will be=4X36° = i44° (Fig. 122, Plate LXI). 

We find in the trigonometrical records, or from direct ob- 
servation at the station Accampamento Reale: 

Horizontal angle: Cian del Lei-Accampa- 

mento-Nomenon =135° S8' 23".2S 

Horizontal angle: Nomenon- Accampamento- 

Ruja = 13° 51' o4".S 

Hence the horizontal angles of orientation for the fifth plate: 

:^ w' = (PS - F -Nomenon) = 8° 01' 36".7S 

^ 0) =(Nomenon — F— Ruja) — a/= 5° 49' 27".75 

13° 51' o4".so 

In the computation for the abscissae x and x' imder Example 
No. V we had found 

a/=go°-d='j° 53' oi".3 
and CO =90°- ^ = 5° 58' 03".!. 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 107 

The differences between these values for w is=o8' 3S".35 and 
for oj' =oW 35".4S, or the difference in the mean value of a and 
0/ is expressed by 

Jw=T8' 3s".4o, 

being about the same error in azimuth of the principal point P 
that had been found for the panorama obtained from the station 
Pimta Percia with the same instrument (considered under 
Example No. IV, where we obtained' iw=±8' 2?>".g). This 
error, Jw=±8' 35".4o, corresponds to a horizontal linear dis- 
placement, Ax, of the principal point P of the photographic per- 
spective: 

Jx= ^0.610 mm. 



5. Application of Franz Hafferl's Method for Finding the Focal- 
length Value of a Photographic PERSPECTrvE from the Ab- 
scissa OF TWO Pictured Terrene Points. 

When the horizontal distances, D and D', are great com- 
pared with the differences in elevation between the pictured 
points under consideration and the camera station, the ordi- 
nates, y and y', will be rather short and the accurate measure- 
ment of their lengths will be difficult. 

In such case it may become advisable to determine the value 
of the focal length / by means of the abscissae, x and x', of the 
pictured terrene points. To do • this L. P. Paganini uses the 
following method, suggested by Franz Hafferl of Vienna. 

We have with reference to Fig. 129, Plate LXIV: 

00'= horizon line of photographic perspective; 

Vs and Vs' = horizontal directions from the camera station V to the 

pictured points i and s' ; 
FP= perpendicular to the horizon line 00'. 

It is desired to find the length value for /. 



I08 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Describe a circle through the three points V, s, and s', the 
center of which may be at C. The angle sCs' is double the 
angle sVs' and the perpendicular CM to the line ss' will divide 
this Une and also the center angle sCs' into two equal parts; 
hence 

^sCM==:!fs'CM=4sVs'. 

If R be the radius of the circle described through the three 
points s, s', and V, we will have, from the triangle sMC, the 
following relation : 

_ „ SM x+x/ I „,, x+x' 

sC=R=-. — 7}= ^T/, as SM= . 

sm V 2 sm V 2 

After having drawn the diameter mn parallel to ss' we find 
f=^VP = VA+AP. 

VA being vertical to mn it will be the middle proportional to mA 
and An: 

mA:AV=AV:An, 

mA-An = {AVf. 

We can replace mA by (mC-AC)=R ^^; AC=SM-SP; 

and as An=nC-\-AC=R-{ , 

2 

we will have AY=^(r-~^ (^+^) 

and finally ^P=CM=5M cot F=^^^ cot- F. ' 

2 

Example No. VI. — Determination of the- value / for a plate 
by means of the abscissae of two pictured points. 



PHOTOTOPOGRAPHIC SURVEYING^ METHODS. I09 

From the data derived from the computation in Example 
No. IV we find 

F=28° 02' 30" 

x= 46.05 mm. and »' = 75.4omfn. 

:j(;+ii(/ = 121.45 mm. »'—»; = 29.35 mm. 

= 60.725 mm. = 14.675 mm. 



Computation of R— -. — 7^: 



log -^^ = 1.7833675 
colog sin 7=0.3277972 



logi? = 2.iiii647 
i? = i29.i7i mm. 



Computation of F4 = >J (i? + — ^j \R ^j : 



R^- =143.846 mm. 

R =114.496 mm. 



log (ie+^) =2.1578978 

log [R —J =2.0587903 

log yl^ =4.2166881 

log VA =2.1083440 
VA =128.335 mm. 



no PHOTOTOPOGRAPHIC METHODS AND INSTRUMENT^. 

x+x' 
Computation oi PA= cot V: 

log -^ = 1.7833675 
log cot 7=0.2735641 



log P^ =2.0569316 
PA = 114.007 mm. 

Hence }=VA+PA = 242.342 mm., 

which compares very closely with 

/ = 242.332 mm., 

obtained under numerical Example No. IV. 

In practical work, like the great Italian topographic sur- 
vey, it would take too much time and labor to determine the 
focal length (/), after the method just shown, for each perspec- 
tive, or even for each panorama set. If there is no reason for 
doubt that the optical axis of the camera intersects the picture 
plane at right angles (as it does for the Itahan phototheodolite 
with a sufficient degree of precision) it will be more simple to 
determine the value / for an entire panorama set (and also for 
all subsequent panoramas that may be executed with the same 
objective, and with the same focal length, which may be veri- 
fied at each exposure) by simply checking the scale-reading on 
the objective tube, which should remain the same for all pictures. 
Under this supposition the focal length is computed in the fol- 
lowing manner: 

Since the azimuthal swings of the camera after each exposure, 

P'FP2, pwp^ psyp*..., 

are aU alike,-and each being equal to 36° (Fig. 122, Plate LXI), 
the angles 

P'Vm, mVP^, PWm' . . . 



PHOTOTOPOGRAPHIC SURVEYING METHODS. IIP 

"will each be = i8°. If x'" denotes the maximum length of the 
abscissae of the plates then 

x"^ = P'm=mP^=P^m' = . . . =/tan i8°, 



hence /= 



tani8°" 



In the preceding (page 93) it has been stated that two adjoining 
negatives of a panorama set have a vertical marginal strip of the 
pictured terrene in common, and the width of this strip may 
be expressed in arc by the angle pVq (Fig. 122, Plate LXI). 

If the negatives are sufficiently clear (showing a good defi- 
nition) it will be an easy matter to locate a point m, either on 
the negative or on the photographic print, that may be identi- 
fied on both overlapping strips pq (Fig. 122, Plate LXI) of two 
adjoining plates P' and P^, which will be on or near the horizon 
line 00', and distant from the principal Hne //' of plate P' by 
mP', and distant from the principal line of plate P^ by mP^, mP' 
being = wP2- 

If we now select such points w', m", m'" . . . , that can be 
readily identified upon two adjoining perspectives, 

P2 and P3, P3 and PS P* and Ps . . . , 

we will obtain ten values for m for the entire set, and the focal- 
length, /, for the panorama may be determined by means of 
the preceding formula, 

x^ 
'^ tan 18°' 

where x'" is the arithmetical mean of the ten greatest abscissje 

P'm, mP^, P^m' .... 

Example No. VII. — By means of ten negatives of a pano- 
rama station, obtained with Paganini's phototheodolite, described 
in "La Fototopografia in Italia," the following values were 
found for the distances x'": 



112 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



xm for pi _p2 =7y.io mm. 



X™ 



p2 _p3 =77.15 

pa _p4 =77.00 

pi _p5 =77.40 
pS _p6 =77.40 
p6 _p7 =77.20 
p9 -P10 = 77.40 
piO_pi =76.90 

log 77-194= 1-8875835 
colog tan 18° =0.4882240 



a;'" = 7 7. 1 94 mm. = mean value. 



log/ = 2.3758075 
/ =237.6 mm. 

The above values were obtained by using the negative plates 
and reading the measurements scaled off (by means of dividers) 
on the graduated rulers of the graphical instruments (" icono- 
meters ") of the Royal Military Geographical Institute, 

Using the positives (albumen prints) of the same panorama 
the following results were obtained: 

xm for pi — p2 =76.25 mm. 



xm 

X"' 

x'" 



P2 _p3 =76.20 ' 
P3 _p4 =76.10 ' 

P9 -pio = 76.7o ' 

piO_pi =76.00 ' 

log 76.25 = 1.8822398 
colog tan 18° =0.4882240 



■ :x;"» = 76.25 mm.=mean value. 



log / = 2.3704628 

/= 234.67 mm. = 234.7 mm. 
The negatives gave as" = 7 7. 19 mm. 
The positives gave 5(;'" = 76.25 mm. 

Diff. = 0.94 mm. 

The evident contraction of the greatest abscissa — amounting 
to very nearly one millimeter on the prints — is due to the shrink- 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 



"3 



age in the 24X18 cm. albumen paper. Whenever " positives " 
(prints) are used in the iconometric map construction, this shrink- 
age should be ascertained and taken into account. Of course, 
the elements of the " contracted " photographic perspectives are 
now substituted for those of the glass negatives. 



6. Supplement. 
I. Forms Showing Arrangements of Field Records for Panorama Views. 



Station 



on Punta Bivula (trigon. pt.)i on the rid 
of the Valsavaranche and Rhfimes. 



;e between the valleys 



Date, 



September 18, 1884. 



Orientation of the 
Panorama. 


Perspectives 
belonging 

to the 
Panorama. 


Directions 
to the Prin- 
cipal Points 
of View. 


Focal 
Distance. 


Remarks. 


Punta Gran Paradiso, 
78° 27' 00" 


p2 

pa 
P* 

p5 


78° 27' 
114 27 
150 27 
186 27 
222 27 


244.5 ™n^- 

> 

c 
9_rt 


Time 

iqS' 

los 

9 = 

125 

9N 


of exposure: 

with small- 
e s t dia- 
phragm. 
No. 7 




pfl 


258 27 


Jf^ Dh 


108 "1 




Punta della Grivola, 
123° 47' 00" 

[ 


. pi 

p8 

po 
pio 


294 27 
330 27 

6 27 
42 27 


t55 


9" 
108 
los 
los 


1 Fine w e a - 
*■ ther 


Directions and Vertical Angles of the 
Trigonometrical Points. 


Computation of Elevation of Station and 
Elevation of Line of Horizon. 




Station on partly removed signal. 
Elevation of instru- 




ment = 2 . 30 m. 
Geodetic point, ele- 
vation = 3413.69 m. 




Elevation of lines of 
horizon of the 




panorama =3415 .99= 3416 m> 



(The adjoining page of the record book may be utilized for topographic 
sketches from station, for detailed remarks, names of roads, etc.) 



114 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



Station 



on Punta Percia, on ridge between the valleys of the Valsava- 
ranche and the Rhemes. 



Date,. 



September 19, iS 



Orientation of the 
Panorama. 



Perspectives 
belonging 

to the 
Panorama. 



Directions 
to the Prin- 
cipal Points 
of View. 



Focal 
Distance. 



Remarks. 



Punta deir Erbetet, 
282° 04' 00" 



p2 

ps 

pA 

pf, 

P' 
pi 

ps 
po 



170° 
206 
242 
278 
314 
35° 
26 



62 



00' 
00 
00 
00 
00 
00 
00 



244.5 inm. 



Time of exposure: 



9» 00 
184 00 



6= 

8» 

9« 

10' 



oS\ 
7M 



' Shorter e x - 
posure than 
before on ac- 
count of the 
great reflec- 
tion from sur- 
rounding 

. glacier. 

, Diaphragm 
No. 7 

Fine weather 



Directions and Vertical Angles to the Sur- 
rounding Trigonometrical Points. 



Computation of Elevation 
Elevation of Line of 



of Station and 
Horizon. 



Cima di Breuil 
Elevation 

Punta dell' Erbetet 
Elevation 



220° 54' 00" 
I ^3 00 

282 04 10 
3 36 30 



Cima di Nomenon 222 42 00 
Elevation i 38 30 



Elevation of Invergnan= 3607 . 72 ra. 
Diflf. of elev. -f corr. = 406.15 



Elevation of Nomenon ■■ 
Diff . of elev. -I- corr. 



Cima di Rouletta 
Elevation 



o 44 30 
3 II 3° 



Punta deir Invergnan 80 07 00 
Elevation 3 42 00 



Cima di Toss 
Elevation 



34 II 30 
o 30 30 



Elevation of Toss 
Diff. of elcv. -1- corr. 



Elevation of Breuil 
Diff. of elev. + corr. 



Elevation of Rouletta 
Diff. of elev. -f- corr. 



Elev. of line of horizon= 



3201 


.S7 


3488 


42 


284 


94 


3202 


48 


3302 


24 


99 


84 


3202 


40 


34,';4 


62 


252 


64 


3201 


98 


33«4 


10 


182 


28 



3201.82 
3202.30 



PHOTOTOPOGRAPHIC SURVEYING METHODS. 



"S 



II. Form Used for Recording the Elevations of Secondary Points of the 

Panorama Views. 



Names or 

Numbers of 

Points. 



Stations 
whence They 
were Derived. 



Elevations of 
Stations. 



Difference of 
Elevations. 



Elevation of 
Point. 



Remarks. 



D. General Arithmetical Method for Finding the Plotted Positions 
oj Terrene Points when Pictured on Vertically Exposed 
Picture Planes. 

With reference to Fig. 43, Plate XXIV, we have 

5 and 5' = the two camera stations; 
MN and M'N' = two photographic perspectives obtained from 
5 and S' respectively; 
a and a' = two pictures of a point A ; 

/=5P=5'P' = constant focal length for both pictures or plates; 
Z>= 5(^4 0= horizontal distance from 5 to .4 ; 
i)'=5o'4o= horizontal distance from S' to A; 
d = Soao] 
d'=So'ao'; 

5 = 5o5o'= horizontal distance between the two stations 5 and 
S', the elevation ' of A above the horizon plane of the 
station S=H and above the horizon plane of the 
station S'=H'. 

Finally, the horizontal angles included between B and the 
principal planes that pass through the two stations S and 5' = 
tto and ao' respectively. 

If we refer the pictured points to the principal point P of 
the photographic perspective by means of the rectangular sys- 



Il6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

tem of coordinates formed by the principal and horizon lines 
(//' and 00') the coordinates of a on MN will be 

aa'=y, a'P=x, 

and those of a' on M'N' will be 

o'ai'=/, ai'P'=x'. 

If the camera is in perfect adjustment, if the base line B 
has been measured in the field, and if the angles ao and ao' have 
been observed, we know the values of 

B, ao, ao', f, x, x', y, and Y 

(the coordinates are measued on the negatives MN and M'N') 
and we can now compute: 

(i) The horizontal angle 7- (or f) included between the prin- 
cipal ray SP (or S'P') and the horizontal direction Sa' 
(or S'ai') to any point A from the equation 



X / , x'\ 

tan T^~r I or tan T=~r)- 



(2) The vertical angle /? (or /?') included between the plane 
of horizon for the station S (or S') and the Une of direction 
Sa (or S'a') to any point A from the equation 

tan/3=| (or tan/3'=^), 

and as 

rf=V/2+x2 (or d'=\/p + {xff), 
we may write 

ta"^=;7fc2 (or tan/3'=^7^^,). 



PHOTOTOPOGRAPHIC SURVEYING METHODS. I17 

Of the triangle 5o^o>S'o' we know the side SoSo'=B and the 
angles ;-, ao, f, and ao'; hence 

B sin(7^+ao') sin (/ +0:0') 



D sin [180° -{r+ao + f +ao')] sin (r +ao + f +"0') ' 
whence 

B-sm{f+ao') 



D=SoAq = 



sin (r+ao + r'+«o') 

(or Z)'=5oMo = ^-f|^'4±^). 
\ " " sin (;-+ao + r^+ao)/ 



We can now compute from 

„ H 
tan P^^, 

the difference in elevation between A and 5 (or S'), 
H=D tan /? (or W =D' tan /?')• 

£. General Arithmetical Method for Finding the Plotted Positions 
of Terrene Points- when Pictured on Inclined Picture Planes. 

For inclined picture planes we will have to take the angles 
of inchnation of the plates into consideration. Under angle of 
inclination of a plate we understand that angle which is included 
between the optical axis of the incUned camera and the horizon 
plane of the camera station (second nodal point). 

Referring to Fig. 38, Plate XXII, and Fig. 44, Plate XXV, 
we have 

a = horizontal angle included between the principal plane and 
the vertical plane passing through the station 5 and the 
point A, pictured as a; 

/3 = angle of elevation of the point A ; 

)-= angle of inchnation of the photographic plate MN; 



Il8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

5 = complement of 7-=i8o° — j-; 
O0'= horizon line when MN is vertical {00' is permanently 
marked on the camera) ; 
/'= principal point for the vertically exposed plate; 
P;r = )' = ordinate of c, Fig. 44, Plate XXV; 
a;r=x = abscissa of a, very nearly = a' P', Fig. 44, Plate XXV; 
2= vanishing point ("kernel point") for all vertical lines 
pictured on MN. 

From inspection of Fig. 44, Plate XXV, we find directly 

aa' TZTt! np P p-P7i _2_cos_£-/sinj-_ 
tan P-s^>-Sa'~S^' ~^yp. + (Si^f =Vx2 + {Sn ^%T^f 
y cos ;' + /sin ;- ji cos ;- — /sin ;- 

V'x2 + (57r+(07r)2 V'a;2 + (/cosr+3'sinr)^' 
and 

tana = 



Sit' Sn+fm /cos ;-+)'sin ;-" 

For the vertically exposed plates we had found 

« y , X 

tano= , and tanQ:=-r. 

^ \/a;2+/2 / 

The preceding formulas for the inclined plates will assume 
the form of the latter if the angle of inclination y is reduced 
to O, as sin ;- will then become equal to O and cos 7- equal to i. 

After the values for a and ^ (or a' and /?') have been com- 
puted the value for SqAo=D (or Sq'Aq=-D') and for AA'=H 
{or AA"=H') may be obtained as follows: 

Referring to Fig. 38, Plate XXII, we find 

D sin (e' —a') 

B^sm[i8o°-{a + e + s'-a')]' 

B-sin (e'-a') 



hence D = - 



sin {a + e + s'—a'y 



PHOTOTOPOGRAPHIC SURVEYING METHODS. IIQ 

and from -=r=tan/? 

we obtain 

y cos 7- — /sin y 



-H'=Z)tan/3-Z» 



Vx^ + (/ cos 7- +3' sin yY 



If an ordinary surveying camera with constant focal length is 
used, and it should become desirable to expose a- plate in an 
inclined plane, the complement d of the angle of inchnatioii y 
of the optical axis may be more readily (but only approximately) 
determined than y by carefully measuring the distances AD, 
Fig. 45, Plate XXV (in the direction of the line of a suspended 
plumb-bob), and DB, AB, being parallel with the photographic 
plate. 

F. General Analytical Determination of the Elements of a 
Photographic Perspective. 

When in addition to the photographs other data — obtained 
by the necessary instrumental measurements — are given for a 
graphical determination of the focal lengths of the pictures, 
their horizon lines and their principal lines, then these elements 
may also be determined analytically. 

A picture MN, containing the images a, b, and c of three 
known points A, B, and C, may be given and the position of 
the camera station (whence this picture was obtained) may be 
known with reference to the three plotted points A', B', and 
a, Fig. 46, Plate XXVI. 

To orient the picture trace (or the ground line) gg' with 
reference to the plotted station S' and plotted points A', B', and 
C the latter are preferably referred to a rectangular system of 
coordinates (S'Y and S'X, Fig. 46, Plate XXVI) having the 
plotted station S' as the origin. To simphfy matters one of 
the axes of the system may be laid through one of the plotted 



I20 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

points. In Fig. 46, Plate XXVI, the axis of abscissae S'X passes 
through the point c'. 

The coordinates of the points A', B', and C, measured on 
the plotting-sheet, may be 

X\Yi, X2Y2, and X3 respectively. 

The coordinates of the orthogonal projections (on the pic- 
ture trace gg') of the corresponding points pictured on the photo- 
graph MN and located upon the radials S'A', S'B', and S'C 
may be designated by 

xiji, xiiyii, and xm respectively. 

The horizontal distances measured on the photographic 
plate between a and h, between h and c, and between a and c 
(the same as those measured on the picture trace between a' and 6', 
between V and c', between a' and c') may be designated by 

otI, mil, and win respectively. 

From an inspection of Fig. 46, Plate XXVI, it will be evi- 
dent that 

(i) yr.xi=Yi:Xi; 

(2) yiv.xii = Y2:X2; 

(3) 3'i:3'ii=wIII:otII; 

(4) {xni~xi):{xii-xi)=-m^'i'i^:mi-\ 

(5) {xiii-xif + {yif^(minf. 

From these five equations the five unknown quantities of 
xi, yi, xii, yii, and xm — the coordinates of the points to be 
located — may be computed. 

From the area of the triangle S'a'c', 

yi-xiii win 
~~2 '~' 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 121 

we find the focal length 

yi-xiii 



t- 



wfiii 



The angle of orientation ;-, included between the principal 
ray S'P' and the base line S'C, may be derived from the equation 



cos ;-= or 



:«ill will 

The principal point P' may be located upon gg' by laying 
off on the picture trace gg' from c' the length, 

P'c' = xiii sin 7-. 

The differences in elevation between the station 5 and the 
three points A, B, and C being known it will be an easy matter 
to draw the horizon line upon the photograph. 



II. Graphical Iconometrical Plotting Methods. 

A. Col. A. Laussedai's Method (French Method). 

Col. Laussedat's methods of constructing topographic maps 
from (photographic) perspective views of the terrene having 
been widely pubUshed, they form the groundwork for all sub- 
sequent work in this field. They are chiefly of a graphical char- 
acter and in harmony with the laws of perspective. Col. Laus- 
sedat considers two general cases in reconnoitering expeditions 
■where photo topographic methods may be applied with advantage: 
First. The observer may remain sufficiently long in one 

locality to make a survey on a large scale, say i : 20000 

and even larger for special purposes. 



122 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Second. The explorer moves rapidly from place to place, 
gathering only the most necessary data on his itinerary 
to enable him to plot the topography of the traversed 
country as a " running survey" on a small scale — say,, 
1 : 50000 or smaller — preserving and representing only the 
principal topographic features met with on the track 
survey. 

In the first-mentioned case the explorer will measure one 
or more base Hnes with as great an accuracy as the means and 
time at his disposal will admit. He will cover the area to be 
mapped with a system of triangles connected with the base 
lines, and inasmuch as the triangulation stations will also be 
occupied with the surveying camera the scheme should be laid 
out with due reference to the subsequent iconometric plotting 
of the topographic features. 

When applying the ordinary surveying methods the tri- 
angulation scheme would probably be laid out with a view toward 
covering as large a territory as possible with each triangle, occupy- 
ing the smallest possible number of intervisible points. 

With the application of photography, however, the conditions 
become somewhat changed. Every topographic feature that 
is to be plotted iconometrically should be seen from two or 
more camera stations, and as each camera station is to be con- 
nected with the triangulation system, either directly or indirectly, 
the number of triangulation points should be a relatively large 
one. Often it will not be desirable that the highest peaks trigo- 
nometrically laid down on the map should be occupied with 
the camera, especially when fogs prevail in the higher altitudes, 
and when other camera stations would answer the requirements 
just as well. 

Regarding the second case, where the explorer follows a 
certain route without making side excursions and never stopping 
longer in one place than is absolutely necessary for his observa- 
tions, the phototopographic method .becomes even more valuable 
than in the first case, particularly when traversing open and 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 23 

broken country. For this kind of topographic reconnaissance it 
may well be said that the photographic method surpasses all 
other surveying methods regarding the amount of data which 
may be collected in the field in a limited time period. 

All topographic operations and instruments serve to measure 
distances and vertical and horizontal angles. A photographic 
perspective of which the elements are known will give all the 
data needed to determine the vertical and horizontal angles of 
lines of direction drawn from the point of view to all points 
pictured on the photograph. 

The points A and B shown on the plate MN, Fig. 47, Plate 
XXVI, may represent the pictures of two mountain peaks. The 
points marked a and h will be their projections upon the horizon 
line HH' . The angle aSb=a will be the horizontal angle of the 
lines of direction SA and SB if S is the point of view on the 
distance line SP. 

The vertical angles /? and f may be shown in horizontal plan 
by revolving the vertical planes passing through SA and SB 
about the lines Sa and Sb, respectively, until they coincide with 
the horizon plane HH', when 

a{A)=aA, 
{A)aS=AaS =^go°, 
^=ASa = {A)Sa = (J^). 

The vertical angles 13 and y may now be measured in horizontal 
plan as (/?) and (7-). 

To indicate in a general way Laussedat's method of icono- 
metric plotting and to show how the plotted features of the terrene 
may be obtained from the photographs we will refer to Figs. 48 
and 49, Plate XXVII, where A, B, and C represent three camera 
stations (plotted in horizontal plan. Fig. 48), whence three per- 
spectives I, II, and III, Fig. 49, of the same knoll D may have 
been obtained. The traces of these three pictures on the plotting- 
sheet may be H^Ha, HbHb, and HcHc- AH three photo- 
graphs having been taken with the same instrument of constant 



124 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

focal length, the distance lines PaA, PbB, and PcC will be 
equally long. 

I. Orientation or the Picture Traces on the Plotting-sheet. 

The three stations A, B, and C are plotted either as parts of 
the triangulation system or by measuring the base line AB on 
the ground and observing the horizontal angles CAB and CBA, 
when the sides AC and BC may be found graphically or by 
computation and the triangle ABC be plotted upon the working- 
plan. 

Horizontal angles or directions to D having also been observed 
from A, B, and C, its position with reference to A, B, and C 
may also be plotted. 

To orient or plot the three picture traces we must know the 
horizontal angles a a, ub, and ac, which are generally observed 
for each picture by means of the horizontal circle attached to 
Laussedat's phototheodolite. 

These angles are plotted from A, B, and C on the lines AD, 
BD, and CD with reference to the position of D on the photo- 
graphs, whether to right or left of the principal line VV. The 
constant focal length = / of the three negatives I, II, and III is 
now laid ofE on the radials APa, BP^, and CPc- Perpendiculars 
erected in Pa, Pbi and Pc to the Unes APa, BPb, and CPc re- 
spectively, will represent the picture traces HaHa, HbHb, 
and HcHc. The abscissas P^c^a. -Pb^b. and Pcdc, measured 
on the negatives I, II, and III, should be made equal to the 
distances P^t/yi, PBdBj and Pcdc on the picture traces. 

The point D is termed a " reference point," and every picture 
that is to be used for iconometric plotting should contain the 
image of at least one such reference point of known position in 
both the horizontal and vertical sense. 

2. Locating Points on the Plotting-sheet that have been Identified 
ON Several Photographs. 

After the picture traces have been oriented any (other) point 
T of the terrene shown on two or more pictures may readily 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 12$ 

be plotted without requiring additional instrumental measure- 
ments in the field. 

To locate the plotted position of the point T, Fig. 48, Plate 
XXVII, shown as Ia and tc on two pictures I and III, Fig. 49, 
Plate XXVII, the abscissae Pa^a and Pete are laid off on the 
picture traces HaHa and HcHc', respectively, from Pa and Pc, 
Fig. 48, and on the side of P corresponding with the position of 
the image / with reference to the principal line VV, Fig. 49, Plate 
XXVII. 

Lines drawn from A and C, Fig. 48, through i^ and tc will 
represent horizontal directions to T from the stations A and C; 
their intersection at T will locate the position of the tree in 
horizontal plan. 



3. The Iconometkic DEXERMiNAnoN of Elevations of Pictured 
Terrene Points. 

The horizon line HH' of a perspective view. Fig. 47, Plate 
XXVI, being the intersection of the horizon plane vnth the ver- 
tical picture plane, will intersect points in the picture which in 
nature have the same elevation as the optical axis SP of the 
camera. All pictured points falling above the horizon line are 
higher and all points falling below the horizon Une are lower 
in elevation than the point of view S. 

The distances Sa and SA, Fig. 50, Plate XXVII, are measured 
on the plotting-sheet and the ordinate {Aa, Fig. 47, Plate XXVI) 
of the pictured point a (its distance from the horizon line) is 
taken from the negative. Perpendiculars to 5.4 are then erected 
in a and A (on the plotting-sheet. Fig. 50, Plate XXVII), and 
the one in a is made equal to the ordinate {Aa, Fig. 47, Plate 
XXVI) of the pictured point = a(a). Fig. 50, Plate XXVII. 

If we now draw the line S{a) to its intersection with the per- 
pendicular to 5.4 in A, the triangle Sa{a) and S{A)A will be 
similar and the angle AS{A) will represent the vertical angle 
of the visual ray from S to A, revolved about 5.4 into the plane 



126 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

of the horizon. From the similar triangles Sa{a) and SA{A) 
we derive the proportional equation 

A{A):SA=a(a):Sa, 
whence 

a(a)-SA 



A(A)^- 



Sa 



a(a) is measured on the negative; SA and Sa are taken from 
the plotting-sheet. A{A) measured on the plotting-scale will 
give the difference in elevation between 5 and A. 

In practical work the elevations of the camera stations are 
known and by adding the height of the instrument including the 
value for A{A) to the elevation of the camera station the absolute 
elevation of the geodetic point A is found, which, however, is still 
to be corrected for curvature and refraction. 

A second value for the elevation of the geodetic point A is 
found in the same manner from another negative containing 
an image of A and obtained from another station. The mean 
of several such determinations is adopted for the final value for 
the height of A. 

4. Drawing the Plan Including Horizontal Contours. 

After some little practice, points pictured on different nega- 
tives but representing identical geodetic points will readily be 
identified by the observer and he will select characteristic points 
to reproduce the watercourses, water-sheds, roads, shore lines, 
etc., on the plotting-sheet. 

After these principal guide lines are well located on the chart 
the buildings, outlines of woods, marshes, etc., are plotted, includ- 
ing everything that is to be shown on the finished map. 

Enough points must be plotted iconometrically to form a 
good control for a correct delineation of the relief. Should the 
number of points determined on the plan be sufficient only to 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 27 

give an adequate control for the delineation in the horizontal 
sense, additional points should be plotted from the photographs 
to obtain an equally good control of the terrene in the vertical 
sense. 

The planimetric work completed, elevations of as many of 
the plotted points as seem necessary are determined and inscribed 
on the chart. Horizontal equidistant contours may then be 
drawn by interpolation to harmonize with the elevations sufi5xed 
to the points of control on the chart, conforming the courses of 
the contours between the determined points to the configuration 
of the terrene as it is shown on the panorama views. 

It cannot be denied that a certain amount of study and 
practical application are requisite to enable the iconometric 
draughtsman to interpret forms correctly when shown in per- 
spective. Yet it should also be admitted that such transla- 
tion or conversion of the configuration of the terrene into hori- 
zontal projection may be accomplished far more accurately 
at one's leisure in the office by means of geometrically correct 
perspectives than could be accompUshed by sketching in the 
field. 

When topographic features as seen from one direction are 
sketched by the plane-tabler, their forms will often be foimd to 
have been misconceived when they are again seen from another 
point of view. Of course, forms sketched on the plane-table 
sheet may then be corrected, in a measure at least, but many 
details are sketched that will not be seen again from other sta- 
tions, and even those that are again observed from other stations 
may not be modified to conform with their true shapes unless 
the original station whence they were first seen and sketched 
■ could again be occupied to verify the suggested changes, and, 
generally speaking, topographers regard a second occupation 
of a station with little favor, considering it too great a loss of 
time, retarding progress and considerably increasing the cost 
of the work. 

In iconometric plotting, however, it would be an easy matter 



128 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

to refer back to the panorama views obtained from some other 
station, and the plotting of topographic details should not be 
attempted without having first made a careful study of (and a 
close comparison between) the various pictures representing 
identical areas but seen from different points of view. 

B. Dr. A. Meydenbaur's Method (German Method). 

The pantoscopic lens (made by E. Bush in Rathenow, Prussia) 
of Dr. Meydenbaur's surveying camera commands an angle of 
about ioo°. By excluding the external rays of the effective 
field of these lenses by means of diaphragms (within the camera) 
pictures are obtained subtending a horizontal angle of but 60° 
(irrespective of the 5 mm. wide margins with which two adjoining 
plates lap over each other) requiring six plates for a complete 
panorama. 

After the camera has been adjusted over a station the pano- 
rama is photographed by exposing six plates in succession, each 
successive turn of the camera in azimuth covering an angle of 60°, 
two adjoining plates lapping oveT each other by a margin of 3° 
in arc, Fig. 51, Plate XXVIII. These common margins, con- 
taining identical sections of the panorama view, may well serve 
to find the value for the focal length of the negatives. 

From the panorama set of six plates exposed from one sta- 
tion objects or geodetic points may be selected on the middle 
lines of the common margins of adjoining plates that must be 
equidistant from the principal lines of adjoining plates. 

I. Determination of the Focal-length Value for the Photographic 

Perspective. 

After having selected a series of such reciprocal points, using ' 
a magnifier of low power if needed, on all six plates, we shall 
have twelve determinations (represented by the length /) of 
the positions of the principal line and the greatest discrepancy 
between any two values should not exceed 0.2 mm. if the instru- 
ment is well adjusted. The sum 2/ of two such distances 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 29, 

represents the effective length of one picture, or the length 
of one side of a regular hexagon, with an inscribed circle of 
the radius equal to the constant focal length = / of the negatives. 
The value for the focal length may be found graphically or it 
may be computed from the formula 

; ' 



tan 30° 



When "positive prints" are to be used in the iconometric 
map construction this focal length often will have to be changed 
to correspond with changes that may have taken place in the 
dimensions of the prints compared with their negatives. The 
total linear changes in a print, measured in the direction of the 
principal and horizon lines, may readily be found by comparing 
the distances between the " teeth " (metal plates permanently 
marking the principal and horizon lines in the image plane of 
the camera) on the negative with those included between their 
contact prints on the positive. 

With reference to Fig. 52, Plate XXVIII, we have: 

a6= original length of horizon (or principal) line between the 
teeth of the camera or between their imprints on the 
negative; 

fl'6'=the corresponding length measured on the positive; 

CO = / = constant focal length of the camera or negative. 

The focal length c'O of the contracted (or expanded) posi- 
tive may be found graphically by drawing the triangle abO, 
placing the line a'V (measured on the positive) parallel with ah • 
and moving it (maintaining its direction parallel to ah) towards ■ 
(or from) O until a' falls upon aO and h' upon hO. 

c'O will be the focal length to be apphed v/hen considering' 
the horizontal angles deduced from the positive. Had ah been 
measured in the direction of the principal Une, c'O would be 
the focal length for the positive to be considered when deducing 
vertical angles from the point. The focal lengths c'O should be: 



130 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

ascertained for every print that is to be used in the iconometric 
map construction. 

The topographic map proper is constructed iconometrically 
irom the negatives and positives in a manner very similar to 
that described under Col. Laussedat's method. 

Referring to Fig. 53, Plate XXIX, we have: 

I and 11= negatives of plates exposed at stations / and // re- 
spectively. I shows the image of a signal at station 
//, and negative II shows the image of a signal at 
station 7; 
1 11 = base line measured between the two camera stations / 
and II. 

Both negatives show the image t of the same tower T. 

2. Orientation of the Picture Traces on the Plotting-sheet. 

After the base line I II, Fig. 53, Plate XXIX, has been plotted 
in reduced scale we describe circles about / and II with the radius 
equal to the constant focal length of the negatives, 

cO=f, 

and produce the line / // beyond both station points, make 

I Ho = Olio (Plate I) 
and II Io=0 /q (Plate II), 

describe arcs from IIo as center with IIoC^x]^ (Plate I) and 
from /o as center with IoC=x^l (Plate II) as radius. 

cllo will be the trace of picture I and do will be the picture 
trace of II oriented at station II with reference to the base line 
I II (Plate XXX). 

Fig. 54, Plate XXX, illustrates a more simplified way of orient- 
ing the picture traces. 

After the base line I II has been plotted the horizontal angles 
a I and a" (azimuthal deflections of the optical axis from the 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 13I 

base line / II for the negatives I and II) are plotted at / and // 
with reference to their positions in regard to the principal planes 
at the stations I and // as shown in the negatives I and II (whether 
the station's image falls to the right or to the left of the principal 
line). 

The constant focal length = / of the negatives is laid off on 
the principal line =c'0 for negative I and = c"0 for negative II. 
The images of the stations are projected upon the horizon lines, 
Sii upon HiHi (Plate I) and Si upon HuHn (Plate II), when 

c^OS\\=a' ^horizontal angle included between the principal 

, plane and base line I II, and 

<;"05i = a" = corresponding horizontal angle for station II. 

These angles, a' and a", are transferred from the negatives I 
and II to their corresponding ends' of the base line / II, as indi- 
cated in Fig. 54, Plate XXX. Now lay off the focal length / 
from the base stations / and II upon the sides of the angles a' 
and a" =Ic' a,nd= lie" respectively, and erect perpendiculars 
H'H' and H"H" to Ic' and lie' in c' and c" respectively. They 
will represent the traces in horizontal plan of the vertical picture 
planes / and // in correct position and orientation with reference 
to the base line ///. The remaining two sets of five plates 
each of the panoramas at the stations / and // are easily oriented 
and plotted ; the next plate in order at station II, for instance, 
would have the principal ray (optical axis) in the direction 
(a" +60°), the third (a" + 120°), etc., Fig. 55, Plate XXXI. 
Every plotted camera station will be surrounded by a regular 
hexagon the sides of which represent the picture traces of the 
six negatives forming the panorarfla set for the station. 

3. Locating Points, Identified on Several Photographs, on the 
Plotting-sheet. 

The horizontal locations of all points identified on two or 
more plates are plotted by locating the intersections T of the 
lines of horizontal directions li', II i' , III t'" . . . , Fig. 54, 



132 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

Plate XXX, in the same manner as has been described for Col. 
Laussedat's method. 

4. The Iconometric Determination of Elevations of Pictured 
Terrene Points. 

The elevations of points iconometrically plotted are found 
in the same way as described for Col. Laussedat's method. If 

the scale of the map is r? we will have, Fig. 54, Plate XXX, the 

elevation of station / above II=H = I{Si). 

I(Si)=M-y,-^jj. 

The values of ye=Si{Si), I II and SiII^l" are found by 
direct measurement with a smaU ivory beveled scale divided 
into 0.5 mm., of which o.i mm. may well be estimated after some 
little practice. 

C. Capt. E. Deville's Method {Canadian Method). 

This so-called Canadian method has been in use under the 
auspices of the Canadian Department of the Interior since 1888. 
Capt. Deville, Surveyor-General of Dominion Lands, has given 
a detailed account of his methods in "Photographic Surveying," 
pubhshed at the Government Printing Bureau in Ottawa in 1895, 
and the following paragraphs have been largely taken from 
Deville's book. 

I. General Remarks on the Field-work. 

The area to be surveyed is covered with a triangulation net, 
preferably before the phototopographic work is begun, and a 
secondary or tertiary triangulation, if needed, is carried along 
with the phototopographic work to locate the camera stations, 
in both the horizontal and vertical sense, with reference to the 
primary triangulation stations already established. 



GRAPHICAL ICONOMETEICAL PLOTTING METHODS. 1 33 

The surveyor makes a plot of the entire triangulation covering 
his territory in the field, and he locates on the same all the stations 
that he may occupy to enable him to recognize the weak points 
in his scheme and to plan his operations with a thorough under- 
standing and to secure a good assurance of success. The instru- 
mental work in the field is done merely to locate the camera 
stations and certain reference points (used for the subsequent 
orientation of the picture traces), all topographic details being 
deduced from the pictures. ^ 

The camera stations are located either by angles taken from 
the station to surrounding triangulation points (resecting, three- 
point problem), or by angles observed from the latter to the 
camera station (intersecting, concluding), or by both methods 
com bined. 

The strength or value of the work depends very much upon a 
judicious selection of the points that are to be used as camera 
stations, in order to bring the enitre terrene under proper control 
and to be enabled to construct the map by the method of inter- 
sections of Hnes of direction. Other methods for plotting topo- 
graphic features and details being employed only when the method 
of intersections fails on account of insufficiency of data to give the 
requisite number of horizontal directions (the camera stations 
not being well situated) for a good location of points by intersec- 
tions. 

Each camera station should be marked with a signal before 
leaving it, not be shown on the pictures, but to be observed upon 
with the transit or altazimuth from the triangulation stations, 
in order to locate the position of the camera station upon the plot- 
ting-sheet. 

Frequently it will be of advantage to set the camera up eccen- 
trically over a triangulation station, in order to include certain 
parts of the landscape in the views. The position of the eccentric 
camera station with reference to the triangulation point can 
readily be ascertained (by azimuth and distance) and should always 
be recorded. 



134 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Complete panorama sets are not taken at every station ; it is 
preferred, rather, to increase the number of stations, often occupy- 
ing a special station to obtain a single view only, if by doing so 
better intersections for the location on the plan of some special 
feature may be obtained. 

Multiplicity of stations demands but a small increase in labor, 
either in the field in the extra observations of horizontal direc- 
tions for their location or for plotting them in the office, and 
enough stations should always be occupied to give a full control 
of the relief of the area to be surveyed. 

A certain section of the terrene may be so located that it will 
be a difficult matter to select more than one station, whence it 
may be seen. In such a case the method of "vertical intersec- 
tions" may often become useful; two or more views of such area 
may be taken from stations at different elevations: the greater the 
difference . in altitude between such stations the longer will the 
base line be, and the better will be the intersections which 
locate the features in question (if the latter are not too distant). 

As enough plates should be exposed to completely cover the 
ground, the camera stations will have to be distributed in such 
a way that all valleys, sinks, and depressions that may be repre- 
sented in the scale of the map are well controlled (i.e., are seen 
from different camera stations). It is evident, therefore, that 
the number of stations to be occupied for the topographic develop- 
ment of a certain area will depend upon the character of the 
terrene and upon the scale of the chart. 

Two or three well-defined points (" reference points ") in 
each panorama section (covered by one plate) are observed 
with the transit or altazimuth, noting the vertical and horizontal 
angles upon the outhne sketch that is made for every exposed 
plate. 

These sketches serve far better to identify points with cer- 
tainty than a mere designation (by name or symbol) or descrip- 
tion of the points observed upon. The general triangulation 
notes are kept in the usual manner. 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 35 

Vertical angles are observed to check the position of the 
horizon line on every photograph and to correct errors due to 
small changes in the level adjustments of the camera that may 
arise during the transportation of the instruments over a rough 
trail. 

The horizontal angles are needed, both for the location of 
the camera stations and for the orientation of the pictures (pic- 
ture traces) on the plotting-sheet for the subsequent map con- 
struction. 

2. General Remarks on the Iconometric Plotting of the Survey. 

The field notes of the phototopographic surveys made in 
the Northwest Territory of the Dominion of Canada by the 
Topographical Surveys Branch of the Department of the Interior — 
under Capt. E. Deville, Surveyor- General of Dominion Lands — 
are plotted on the scale of i : 20000, but the maps are published 
on the 1:40000 scale with equidistant contours of 100 feet. 

The phototopographic reconnaissance in southeast Alaska exe- 
cuted by Dominion Land Surveyors — under W. F. King, Alaskan 
Boundary Comn;issioner to Her Majesty — was plotted on the 
scale of 1:80000, with a contour interval of 250 feet, and it 
was published on the 1:160000 scale. 

After the triangulation has been computed and the points 
have been plotted, and after the computed elevations have all 
been affixed to' the marked points on the plotting-sheet, the tri- 
angle sides of the secondary triangulation scheme executed 
during the phototopographic survey are computed, the cor- 
rections to the horizontal angles, indicated by the closing errors, 
having been applied. The latitudes and departures from every 
secondary point to the nearest primary station are then com- 
puted and the secondary stations are plotted by their latitudes 
and departures (unless the primary sides are- too long). 

All camera stations not already included in the secondary 
triangulation scheme are now plotted with reference to the tri- 
angulation points, using either a table of chords or a station- 



136 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

pointer (vernier protractor). If many points had been observed 
upon from the camera station, the horizontal angles are pref- 
erably laid off on a piece of tracing cloth or paper and this 
improvised multiarm protractor is used, hke a station-pointer,, 
to locate the plotted position of the camera station. 

The surveyor should endeavor to obtain at least one direction 
from a triangulation station to every camera station; the plotting 
will then be less troublesome and far more accurate. Photo- 
graphs should not be used for plotting the positions of camera 
stations ; enough angles should always be observed in the field 
to locate (trigonometrically) every occupied station in the manner 
just indicated. 

From the original negatives copies are made, enlarged to 
9^ by 13 inches, on heavy bromide paper (more recently so- 
called " Platino Bromide " paper has been used by Capt. Deville). 
The enlargement adopted in Canada for these bromide prints 
is about 2.1 times, which ratio was selected to utilize the full 
width of the paper found in market. These enlargements, 
being extensively used in the map construction, should be made 
with great care to reduce distortion to a minimum. 

After the prints have been developed (with iron oxalate), 
well washed in acidulated water and fixed, they are again thor- 
oughly washed and dried in a flat position, under special pre- 
cautionary measures to control the contraction or expansion, in 
such a way that the final size of the dry-prints have uniform 
dimensions. Shght distortions that would arise from a play ol 
the negative carrier in the enlarging camera or from the bromide 
paper not lying perfectly flat on the surface of the copying-screen 
are best reduced by using a copying-lens of long focus. 

Before using the prints for the map construction any distor- 
tion due to the enlarging . process should be ascertained, which 
is best done in the following manner: 

Fig. 56, Plate XXXI.— Join the middle notches H and H', 
P and P', and with a set-square test these two lines for perpen- 
dicularity. Take with a pair of dividers the distance of the 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 37 

two notches A and B, which should be one half of the enlarged 
focal length and equal to the distance between the two notches C 
and D. Apply one of the points of the dividers in P and the 
other should come in E and F. Transfer the point to P' and 
check P'G and T'J. If the print stands all these tests, it may 
be used iconometrically; if it does not, it is returned to the pho- 
tographer with the request for a better one. 

3. Orienting the Pictuhe Traces on the Working-plan. 

Every photograph contains at least one, generally several, 
of the triangulation points plotted on the working-sheet and 
the traces of the picture and principal planes are oriented and 
plotted on the plan as follows: 

The distance or principal line PS, Fig. 57, Plate XXXI, 
is made equal to the focal length and the pictured point a of 
the reference point A is projected upon the principal line ( = a') 
and upon the horizon Une ( = a). 

If 5^1, Fig. 57, Plate XXXI, is the plotted position of the 
camera station on the plan, and if 5i^ 1 represents the horizontal 
direction to A from the station 5, make 5iai equal to Sa (taken 
from the photograph on the " photograph-board ") and from oi 
as center, with aa' = Pa as radius, describe an arc to which 
Sip is drawn tangent. Sip will be the trace of the principal 
plane (or the distance hne) and the perpendicular to Sip through 
ai=pai will be the picture trace. Instead of making this con- 
struction on the " photograph-board " (which will be described 
under section i) it may be made on the plan. 

On SiAi take SiB, Fig. 58, Plate XXXI, equal to the focal 
length, erect BC perpendicular to SiAi and make it equal to 
the abscissa { = aa', Fig. 57) of the reference point. Join 
SiC and take Sip equal to the focal length; at p erect a per- 
pendicular to SiC and it will be the trace of the picture plane, 
SiC being the trace of the principal plane. 

Another simple method for orienting the picture trace of a 
photograph having the image of a reference point C is as follows 



138 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

(Fig. 59, Plate XXXII) : Take a triangle of hard rubber or wood 
and mark off along one side the focal distance SP, Fig. 57, Plate 
XXXI, equal to ab, Fig. 59, Plate XXXII, from the right-angle 
comer a. Carefully notch the triangle side at b so that the cen- 
ter of a fine needle marking the plotted station point will fit 
into the notch. From the photograph take the abscissa {a' a = +a, 
Fig. 57, Plate XXXI) of the pictured reference point (a) between 
the points of a pair of dividers, move the triangle about the needle, 
marking the plotted station b with the left hand until ac, Fig. 59, 
Plate XXXI, is equal to the distance a'a (abscissa of reference 
point), held between the points of the dividers. The triangle is 
securely held in this position and lines are drawn along the tri- 
angle sides ab and ac. Produce ac beyond a and check the dis- 
tance ac again to be equal to a'a. The line be represents the 
horizontal direction from the plotted station b to the plotted 
reference point C (its image on the negative. Fig. 57, Plate XXXI, 
was a) and we will now have: 

6ff = trace of the principal plane; 

ac = trace of the picture plane; 
a = projection of the principal point on the plotting-sheet. 

The trace of the principal plane = a& is preferably marked 
by a short line only,, bearing an arrow pointing toward the plotted 
station whence the picture was taken, and the principal point a 
is marked to correspond with the designation of the print, and 
it may be remarked here that as few constructive lines as pos- 
sible are drawn on the working-sheet to avoid confusion and 
mistakes (see photograph-board, section 10, page 148). 

4. The Identificatign of Pictured Points in Photographs 
Representing Identical Points of the Terrene. 

The survey being plotted, analogous to a plane-table survey, 
mainly by intersections of horizontal lines of direction, points 
controlUng the same area must be identified on pictures taken 
from different stations. When selecting such points on a photo- 
graph, preference should be given to such that best define the 



GRAPHICAL ICONOMETKICAL PLOTTING METHODS. 1 39 

surface or terrene like characteristic points of mountain ridges, 
peaks, saddles, points at the changes of slope, bends in streams, etc., 
each point being marked by a dot in red ink on the photograph 
and affixing a number or symbol to it. It will now be necessary 
to identify as many of these points as possible on other photo- 
graphs covering the same area, marking these also by dots and 
giving identical points the same designation, by number or by 
symbol in red ink, as on the first photograph. 

The identification of geodetic points on several pictures offers 
no serious difficulty, and with some practice as many points 
as may be needed for a full development of the terrene, even 
under different illumination of the pictured areas, may be picked 
out with rapidity and precision. In case of doubt, beginners 
may resort to Prof. Hauck's method, which has already been 
mentioned several times in the preceding pages. 

5. Application of PEOi-. Hauck's Method for the Identification op 
Terrene Points Pictijred on Several Photographs. 

The two photographs, picturing the same areas as seen from 
different stations or points of view, are pinned side by side on a 
drawing-board. The images of the camera stations whence the 
pictures were obtained are kernel points; if they fall outside 
of the limits of the pictures their projections on the picture 
traces may be determined from the plotting-sheet or working- 
plan. The parallels to the principal fines, on which the scales 
are to be placed, are drawn as explained in Chapter IV, para- 
graph VII, section 2, and the scales are fixed in position. 

A fine needle is inserted into the drawing-board through each 
of the kernel points and the loop at one end of a fine silk thread 
is dropped over the needle, the other end of the thread being 
fastened to a small weight by means of a slender rubber band 
(see Fig. 60, Plate XXXII). 

A well-defined point is now identified on the two photo- 
graphs sufficiently far from the kernel points, and one thread 
is jnoved (by taking the small weight in one hand) to pass 



140 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

through the point identified on the photograph; the weight 
is deposited on the drawing-board, holding the thread in this 
position under sUght tension of the rubber band h. 

The same operation is repeated with the other thread and 
the other photograph, when the two threads should intersect 
the scales at the same division mark; if they do not, one of the 
scales is to be moved imtil identical division marks are bisected 
by the two threads. The identification of other geodetic points 
pictured on both photographs may now be proceeded with: 

Having selected a characteristic geodetic point on one of 
the photographs, the corresponding thread and weight are moved 
until the thread bisects that point. Noting the point of intersec- 
tion on the scale by the thread in this position, the other thread 
is now moved to bisect the corresponding graduation mark on 
the second scale. The second thread will then also bisect the 
corresponding image of the same geodetic point on the second 
picture. 

6. Plotting Pictured Terrene Points as the Intersections op Lines 
OF Horizontal Directions. Iconometric Plotting of Terrene 
Points in General ("Horizontal Intersections"). 

After enough pictures have been selected to develop a cer- 
tain area, and the identification and marking of the images of 
corresponding geodetic points have been completed, projec- 
tions of all these points on the horizon lines of the pictures are 
marked and transferred to the straight edge of a strip of paper, 
including in this transfer the marking on the strip's edge of the 
principal point of every photograph. 

The strips are given the same designation as the pictures 
to which they belong (by number or symbol) and they are 
then placed upon their picture traces on the plotting-sheet in 
such a manner that the principal points of picture trace and 
paper strip coincide. They are secured in this position on the 
working-sheet by means of small weights or fine thumb-tacks. 

To plot the horizontal projection of a geodetic point, shown 
and marked on two photographs and marked on the correspond- 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I4I 

ing paper strips, two fine needles are inserted into the plotted 
stations I and //, Fig. 60, Plate XXXII, of the two photographs. 
A fine silk thread is attached to each needle. The other end 
of the thread is connected with a small weight W by means of 
a fine rubber band h. 

The thread looped over station needle I is now moved over 
the paper strip (indicating the picture trace on the plan) until 
it bisects the projection a' of the geodetic point's image. The 
weight is now placed upon the paper, holding the thread under 
slight tension of the rubber band in this position. 

The second thread, connected with the needle in station //, 
is placed over the horizontal projection a" of the image of the 
same point A. The point of intersection A of the two threads 
wiU be the desired position on the plan of the point A. 

After this position of A upon the plotting-sheet has been 
checked by means of another photograph taken from a third 
station III, and containing the image a'" of the point A, its 
plotted position is marked by a dot in red ink and its designa- 
tion, corresponding with that given on the prints, is also affixed 
in red ink. A sufficient number of points having been plotted 
in this manner, and all having been supplied with the same let- 
ters or numerals (in red ink) that had been given their, images 
on the photographs, their elevations are determined and also 
affixed to the points in red ink. Frequently the designation, 
of the point by letter or numeral is added in pencil on the 
working-sheet, to be erased after the elevation of the point has 
been aflftxed to it in red ink. 

In case the strips of paper on the picture traces should over- 
lap, as shown in Fig. 61, Plate XXXII, the part CD of the pic- 
ture trace PQ is marked off on the strip MN lying under it, 
the band of paper PQ is then placed in proper position and the 
marks on its edge are transferred to the line CD. The strip PQ 
is now placed imder MN, the marks on the latter, along CD, 
serving the same purpose as those of PQ. 

When a station, A, Fig. 62, Plate XXXII, falls so close to 



142 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the edge of the working-sheet that the trace QR of the picture 
plane falls outside of the limits of the plan, then the trace AC 
of the principal plane may be produced to B, AB being equal 
to AC=ioca\ length of the picture, and MN, drawn perpen- 
dicular to BC or parallel to QR, will occupy with reference to QR 
the same position as the focal plane of the camera does to the 
image plane of the perspective. 

The direction of a point of the photograph projected in Q 
on the picture trace is found by joining NA and producing 
to the opposite side of A. 

As already mentioned, the intersection of two lines of direc- 
tion, establishing the plotted position of a geodetic point, pic- 
tured on two photographs, should be checked either by a third 
line or otherwise, before the position of such point should be 
accepted on the plan as correct. 

Such intersections may, for instance, be checked by deter- 
mining the elevation of the point from both photographs. Unless 
the point has been correctly plotted, these two heights will not 
agree. This check, however, does not guard against slight 
errors in position. 

A check may also be obtained by drawing a line, on which 
the point may be situated (for instance, the shore line of a river 
or lake), with a perspectograph or perspectometer; still, the best 
check will always be afforded by a third intersecting line of direc- 
tion obtained from a third photograph. 

7. IcoNOMETRic Plotting of Pictured Terrene Points by So-called 
"Vertical Intersections." 

We have seen how the base line between two stations is pro- 
jected into horizontal plan when using the method of intersec- 
tions of horizontal lines of direction, hitherto considered, but 
when two camera stations are occupied at different elevations, 
and not far apart horizontally, to locate geodetic points by inter- 
sections of lines of direction, the so-called " method of vertical 
intersections " may be employed with advantage. 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 43 

With this method the base line — its horizontal projection 
being either too short or more frequently falling into the same 
direction with the distant points to be located by the intersec- 
tions of lines of direction — is projected upon a vertical plane. 

The greater the difference in elevation between the two sta- 
tions the greater will the length of this vertically projected base 
line be and the more accurate will be the iconometric location 
of the points by lines of direction. 

We have, with reference to Fig. 63, Plate XXXIII, two camera 
stations A and B, two photographs Ai^ and 5jv obtained, from 
them and containing the image dA and ds of the identical geodetic 
point D. It is assumed that the horizon plane through the 
lower station B be the ground or plotting plane, and that the 
principal plane of the photograph Ajf h^ the vertical plane of 
projection which is revolved about its trace into the horizon 
plane of B. 

a = horizontal projection of station A ; 
fl5= horizontal projection of the base line AB; 
Jf^BJEir'^B= picture trace of photograph A^ m horizon plane 
of B (plotting-plane) ; 
Hb -H"^' = picture trace of photograph B^i in horizon plane of B; 
aP a' =BPs' = constant focal length of the negatives A if 

and Bif] 
oP^'= trace of principal plane passing through aP/ in 
horizon plane of B. 

To plot the position d' of a point D (pictured in An as dA 
and in 5jv as ds) in the plotting-plane the rays AdA and Bds 
are projected upon the vertical plane (revolved about aP^' into 
the ground plane), when (^1) will represent their point of inter- 
section d projected into that same vertical plane (revolved about 
oPa' into the plotting-plane). 

The ray AdA = AD intersects or penetrates the picture plane 
.4jv at a distance =d Ad' AB vertically above the ground plane 
(above the picture trace or ground line HabH'ab of picture Aj^). 



144 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

This ordinate is laid off upon PA'HyiB=PA'{dA)> when (Ia 
will be the projection on the vertical plane of pictured point d^- 

The vertical through a projected upon the vertical plane is 
represented as a{A), and if we make 

a(A)^PAP'AB (picture .4 at) 

=difference in elevation between the two stations B and A, 

(A) will be the upper camera station A projected into the ver- 
tical plane, and the line (/4)(rf^) will be the projection of the 
ray AdA, or AD, upon the vertical plane (revolved about cPa' 
into the plotting-plane). 

The ray BdB=BD intersects the second picture plane Bjf 
in dB- If we draw through ds (projection of ds on ground line 
HbHb) a perpendicular to aP A'^ds'diB, dig will be the pro- 
jection in the vertical plane of the horizontal projection in the 
picture trace of the pictured point ds- Producing dsdis beyond 
diB and making diB{dB)=dBdB' (measured on the negative Bn) 
will locate at (rf^) the projection of the pictured point dg upon 
the vertical plane. 

The perpendicular to aP^' through B locates the projection 
into the vertical plane = 61 of the plotted station B, hence the 
line bi{dB) will be the projection into the vertical plane of the 
ray BdB=BD. 

The intersection (rfi) of &i((fs) with A{d a) locates the pro- 
jection into vertical plane of the point d, and the horizontal pro- 
jection of the point D (plotted on the ground plan) will be on 
the line {di)d', which is the vertical through d (perpendicular 
to uPa' in our case) passing through {d\) and produced beyond 
di, and either horizontal line of direction ad a or BdB, produced 
to intersect this perpendicular {di)di', will locate the position d' 
(of the point D) on the plotting-sheet with reference to the plotted 
stations A (or a) and B. 

(The location of d' as the intersection of the horizontal direc- 
tions ad A ' and Bds would not be very accurate in our case, 
and far less so for points pictured on the other side of the prin- 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 145 

cipal point P^, the angle of intersection of their horizontal direc- 
tions being even smaller than at d'.) 

The point di' being the projection into the vertical plane 
of the point d' (the horizontal projection into the ground plane 
of the point d), the length (di)di (measured on the plotting-scale) 
will represent the elevation of the point D above station B (or 
above the ground plane). 

8. IcoNOMETEic Determination of the Elevations of Pictured 
Terrene Points. 

Generally speaking, one perspective is insufficient to deter- 
mine the elevation of a point, although there are exceptions, 
Uke the points on the horizon line of a photograph which have 
the same elevation as the camera station. A single photograph 
would also suffice if the distance from the camera station to 
the point to be determined vertically be known; for instance. 
Fig. 64, Plate XXXIV, the horizontal projection d of the point D 
being known, its height H above the ground plane will be the 
fourth proportional to the three known lines Bdi, Bd^B and disids): 

5^1= horizontal distance between the plotted station B 
and the plotted point, measured in the plotting-scale 
of the working-sheet; 
Bd\B =horizontal distance between station B and projection of 
pictured point ds in the ground line HbHb, meas- 
ured on the plan; 
<^ib(<^b) = '^ = ordinate of pictured point (^s; measured on the pic- 
ture plane { = dB'dB, Fig. 63, Plate XXXIII, pic- 
ture Bif), 

and the value for H may be computed from the equation 

Bdi 



H = h 



Bd 



IB 



If we now project the plotted point di and the pictured point dg 
into the principal plane and revolve the latter about the prin- 



146 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

cipal line BP into the plotting-plane, we will have with refer- 
ence to Fig. 64, Plate XXXIV, 

■f*(<^B') = height of pictured point dg above the horizon plane = A; 
((ijs')= pictured point de, projected into the principal plane 
and revolved with the latter into the horizon or plot- 
ting plane; 

((i:')di'= vertical distance of the point d above the horizon 
plane = fl'. 

This height, H, is the fourth proportional to the three known 
lengths Bdi, Bd-is and h; 

£P= focal length of the print =/; 
■P(<^b')= ordinate of the pictured point above the horizon line (to 
be measured on the photograph), and 
Bdi'=}+Pdi', where Pdi'=vertical distance between the plot- 
ted point di and the picture trace HBHs'=did (to 
be measured on the plotting-sheet), 

its value may be found with the aid of an ordinary sector, Fig. 65, 
Plate XXXIV, in the following manner: 

Take with a pair of dividers the (ordinate) distance from 
the pictured point dg to the horizon line (on the photograph) 
place one point of the dividers on the division c of the sector, 
when CO = focal length of the photograph, and open the arms 
■of the sector until the second point of the dividers coincides with 
the corresponding division D of the other sector arm (OD being 
equal to 0C = focal length). Now take with the dividers the 
horizontal distance (di'P=did, Fig. 64, Plate XXXIV) of the 
plotted point di from the picture trace HbHq', place one of 
the points in C and note where the second point of the dividers 
intercepts the scale OC, say at A. Turn the dividers about this 
point A (maintaining the opening of the sector unchanged) and 
place the second point of the dividers upon B on the scale OD — 
B corresponding to A, or 05=0^— when AB, measured on 
the plotting-scale, will represent the height, H, of the point d 
above the horizon plane of the station B. 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I47 



9. ICONOMETRIC DETERMINATION OF THE ELEVATIONS OF PlCTintED TER- 
RENE Points by Means of the So-called "Scale of Heights." 

Another method for determining the elevations of plotted 
points iconometrically consists in the use of the so-called " scale 
of heights," Fig. 66, Plate XXXV. 

Make SP equal to the focal length of the photographic per- 
spective, erect PA perpendicular to SP in P, and divide both 
lines into equal parts. Join the points of division on PA to 5 
and through those of SP draw lines parallel to PA. 

To use this scale of heights with a pair of dividers, take 
from the photographic perspective the (ordinate) distance from 
the pictured point to the horizon line and transfer it to the line 
PA =P[i. The point // may be found to correspond to the line S/i, 
passing through the division mark 9 of the graduation on PA. 
"With a pair of dividers take the vertical distance from the hori- 
zontal projection of the point to the plotted-picture trace (measured 
on the working-sheet) and transfer it to SP to the right or to 
the left of P according to the position of the plotted point with 
reference to the picture trace, whether beyond the picture trace or 
between the same and the plotted station. 

In Fig. 66, Plate XXXV, it is shown as falling between the 
station and the picture trace into m. The line mB, parallel 
with PA, is intersected by Sfi in M, and the distance mM, meas- 
ured on the plotting-scale, will be the height of the point M 
above (or below) the station horizon. 

A scale. Fig. 67, Plate XXXV, is conveniently pinned, some- 
where on the plotting-board, perpendicularly to a line AB; the 
division C of this scale, bisected by the line AB, corresponds 
to the height of the camera horizon. Placing one of the legs 
of the dividers with which the length AB was taken off the 
" sector," Fig. 65, Plate XXXIV, or with" which the length mM 
was taken off the " scale of heights," Fig. 66, Plate XXXV, 
in C, Fig. 67, Plate XXXV, the division D of the scale, coincid- 
ing with the other point of the dividers, will indicate the height 



148 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

of the point above the plane of reference or datum plane This 
height is entered in pencil on the plan, inclosed in a small circle 
to distinguish it from the number of the point. It is checked 
by means of a second photograph, and when the discrepancy 
between several values for the elevation of the point falls within 
the limits of permissible error, their mean is entered in red ink 
on the plan and all pencil figures are erased. 

Any marked difference in the values for the height obtained 
from two photographs would indicate that the two points of 
which the elevations were determined are not identical points 
or that an error had been made in plotting the same or in deter- 
mining its height. 

A third intersection would dispose of the first two alternatives 
and a new measurement of the height will show whether an 
error has been made, or whether the discrepancy is due to una- 
voidable errors. 

10. The Use of the So-called " Photograph-board." 

The various constructions described in the preceding pages 
if made directly on the photographs would obscure many details 
and produce confusion through the intricacy of the auxiliary 
lines. Capt. Deville, therefore, had a special drawing-board 
prepared on which as many of the construction lines are drawn, 
once for all, as would have to be repeated for the different prints 
of uniform size (which were, of course, obtained with the same 
camera). 

This so-called " photograph-board " is an ordinary drawing- 
board covered with tough drawing-paper the surface of which 
is to represent both the picture plane and the principal plane 
(both planes revolved into the horizon plane), and it is used in 
conjunction with the photographic perspectives, using the nega- 
tives when great accuracy is required, or using solar prints for 
general plotting. 

Two lines DD and SS', Fig. 68, Plate XXXV, are drawn 
at right angles to each other; they represent the horizon and 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. I49 

principal lines, while PD = PD' = PS=PS' are equal to the 
focal length, so that D, D', S, and S' represent the left, the right, 
the lower, and the upper distance points respectively. 

The photographic perspective is placed in the center of the 
board, within the rectangle TYOZ, the principal line coinciding 
with SS' and the horizon line with DD', and it is secured in 
this position by means of small thumb-tacks, pins, etc. The 
four scales forming the sides of the rectangle OTYZ serve to locate 
lines parallel with either SS' or DD' on the perspective (with- 
out actually drawing those lines). 

At a suitable distance from D' a line QR is drawn perpen- 
dicular to DD', and on it are laid off, by means of a table of tan- 
gents, the angles formed with DQ by a series of lines drawn 
from D as a center. This scale, QR, is employed when measuring 
the altitudes or the azimuthal angles of points pictured on the 
perspective, as will be explained in a following paragraph. 

From 5 as a center with SP as radius an arc of a circle PL 
is described and the latter is divided into equal parts. Through 
the points of division of PL lines converging to 5 are drawn 
between PL and PD'. The Hnes MN are drawn parallel to 
the principal line, as shown in Fig. 68, Plate XXXV, and these 
lines are all used in connection with the scale of degrees and 
minutes QR. 

The studs of the centro-lineads are fixed in A, B, C, and E, 
the lines AB and CE joining their centers, and those needed 
for adjusting the centro-lineads are drawn and used in the man- 
ner to be explained in Chapter X. 

A square, FGKH, is constructed on the four distance points. 
Fig. 68, Plate XXXV. 

II. IcoNOMETKic Plotting of the Tbace as a Figure's Plane. 

If one wishes to use a perspective instrument for converting a 

figure — situated in an inclined plane of which the perspective 

photograph) is given — into the. projection of the figure into 



ISO PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

horizontal plan it will be necessary to locate the traces of the 
figure's plane in both the principal and picture planes. 

We may distinguish between two cases frequently met with 
in practical work: 

(i) The inclined plane containing the figure may be given 
by its line of greatest slope. 

(2) The incUned plane containing the figure may be given 
by three points. 

First Case. — ^The incUned plane of the figure may be given 
by the line of greatest slope, which may be an inclined 
road-bed, the drainage line of a straight valley (thalweg), 
the surface of a glacier, etc. 

This line of greatest slope may be represented on the plan 
by a line ab, Fig. 69, Plate XXXVI, the altitude of a being known. 

The photographic perspective is pinned to the photograph- 
board, and the ground line ZF is drawn, taking the horizontal 
plane through a as ground plane. 

On the plotting-board aO is drawn through a perpendicular 
to the horizontal projection ab of the line of greatest slope AB, 
and it is produced to its intersections L and O with the prin- 
cipal line 5i^i and with the picture trace XiYi. 

On the photograph pE is made equal to pib, at £ a perpen- 
dicular to XF is erected and produced to the intersection /? with 
the pictured line a/3, representing the line of greatest slope AB. 
If we make pN, on the photograph-board, equal to piO of the 
plan and join N with /3 on the picture, this line iV/? will rep- 
resent the trace of the required plane on the picture plane. If 
pQ is made equal to piL and Q is joined with M, MQ will rep- 
resent the trace of the required plane, revolved about SS', on 
the photograph-board, into the picture plane, the station S 
falling in Z>.. 

Producing MQ to R, DR will represent the vertical distance 
of the station 5 above the plane RM^. 

Second Case. — ^The inclined plane containing the figure 
is given by three points. 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 151 

Take for ground plane the horizontal plane containing one 
of the points, a, Fig. 70, Plate XXXVII, and draw the ground 
line XF on the photograph. Join a on the plotting-sheet to 
the two remaining points and produce th^se lines to their inter- 
sections E and F with the pi cture tracej : On the photograph 
make fiK equal to pE and"3Taw iTL perpendicular to XY; join 
the perspectives a and ^ of the points shown as a and h on 
the plan and produce to the intersection with KL. Take piT 
equal to pF, draw TN perpendicular to XY and produce to 
the intersection N with the line joining the perspectives a and y. 
Join N and L, when NL will represent the trace of the required 
plane on the picture plane. 

Produce LN to O and take pG equal to piO; join a and G 
and make piQ equal to pH. The line MQ will represent the 
trace of the required plane on the principal plane revolved about 
SS' into the picture plane, the station being in D. Here also DR 
is the vertical height of the station above the plane containing 
the three given points. 



12. IcoNOMETEic Contouring. 

After the heights of a sufficient number of points have been 
determined to give a good development of the terrene that is 
to be mapped, the contour lines are drawn in by interpolation 
between the points of which the heights had been established. 

In a moderately rolling country a limited number of points 
of known elevations will suffice to draw the contour lines with 
precision, but in a rocky region, where abrupt changes and 
irregular forms predominate, it is almost impossible to plot 
enough control points to enable the iconometric draughtsman 
to render a faithful representation of the relief of the broken 
terrene, and it is here that a close and minute study of the photo- 
graphs becomes indispensable to modify the courses of the con- 
tours to represent the characteristic features of the terrene. 

The value of photographic views for the cartographic delinea- 



152 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

tion of the topography of a mountainous area is generally acknowl- 
edged by experienced topographers, even when using instrumental 
methods exclusively for all the control work. A minute study 
of the pictured terrene will always be of great aid to the draughts- 
man (when inking the topographic sheet), to draw the contours- 
of which the main deflections had been located instrumentally, 
with a more natural and artistic reproduction of nature's forms, 
than could be attained by mechanically inking the pencilled 
lines as obtained by instrumental measurements and free-hand 
sketching alone. 

Instead of drawing the contour lines at once upon the plan, 
the draughtsman may begin by sketching them on the photo- 
graphs first, following the same rules for their location (by inter- 
polation), as if he were drawing them on the plan, for the image 
of every plotted point is already marked on the photographs 
and its elevation may readily be taken from the working-plan.. 
By adopting this plan he will be enabled to follow the inequali- 
ties of the surface very closely and the perspectives of the con- 
tours thus drawn on the pictures will greatly facilitate the draw- 
ing on the plan of their horizontal projections. They may alsO' 
be transferred to the plan by means of the perspectograph or 
perspectometer if accuracy is to give place to rapidity in the 
map production. 

A sufficient number of tertiary points having been plotted 
by the method of intersections, there will be little difficulty in 
drawing the contour lines by interpolation between such points. 
It may happen, however, that the control points are too few 
in number and too far apart to give a good definition of the ter- 
rene (in a topographic reconnaissance)^ and then it will become 
necessary to resort to less accurate methods for locating the 
contours on the plan. For example, the ridge abed of a mountain 
range, pictured on a photograph as aPyd, Fig. 71, Plate XXXVIII, 
may be divided by the contour planes by assuming it to be 
contained in a vertical plane. 

On the plan we produce the projection ad of the ridge to 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 53 

the intersection F with the picture trace and draw through the 
station Si the Une SiC parallel to ad. 

The photograph having been pinned to the photograph- 
board, take from the principal point on the horizon line PV 
equal to PiC and PG equal to PiF. At G place the scale of 
equidistances perpendicular to the horizon line HH', the division 
at G corresponding to the height of the station, and join the 
marks of die scale (corresponding to the elevations of the con- 
tour planes) to the vanishing point V. 

Having thus located the points of intersection of the ridge 
by the contour planes, their distances (abscissae) from the prin- 
cipal line are now marked upon the edge of a strip of paper and 
their directions plotted in the usual way. The intersections of 
the radials (drawn from 5i to the points marked on the paper 
strip) with ad wiU give the intersections of the contour lines with 
the ridge ad. 

When the mountains have rounded forms showing no well- 
defined ridges, the visible outline, silhouetted on the photograph, 
may be assumed to be contained in a vertical plane perpendicular 
to the line of direction drawn to the middle of the ridge outline, 
or silhouette. 

The construction may be made by drawing, on the photo- 
graph-board, SV perpendicular to the direction SM of the middle 
of the outline. Fig. 72, Plate XXXIX; piMi on the plan is made 
equal to PM, and from the projection a of the summit of the 
mountain a perpendicular ac is let fall on SiMi, which represents 
the projection of the visible outline. It is produced to the inter- 
section N with the picture trace. PQ is taken equal to piN and 
the scale of equidistances is placed at Q, perpendicular to the 
horizon line. The points of division are joined to V, these 
radials are produced to intersect ay, and the plotting of the con- 
tour points along ay is done in the same way as described in the 
preceding case, or the directions of the intersections of ay by 
the contour planes may simply be plotted and the contour lines 
drawn tangent to these directions. 



154 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The horizon line, containing the perspectives of all points 
of the same elevation as the camera station, represents the per- 
spective of a contour line when the horizon plane coincides with 
a contour plane. 

The topographic draughtsman should pay particular atten- 
tion to geologic forms and to the originating causes of the topo- 
graphic features, as without such knowledge the correct inter- 
pretation of such forms by means of contours and a faithful 
cartographic representation of the various terrene forms would 
require the cartographic location of a vast number of control 
points. 

Although the terrene forms often result from the successive, 
or from the combined, actions of many agencies, they will yet 
have similar characteristic shapes when resulting from the same 
causes, and the cartographic representation of such typical 
terrene forms (produced by identical agencies) should also show 
a corresponding characteristic similarity in the contour forms. 

13. The Use op the So-called "Photograph-protractor." 

The angle included between the line of direction to a point 
of a photographic perspective and the principal and horizon lines 
(the altitude and azimuthal angle) is sometimes wanted in arc 
measure. 

The azimuthal angle of the line of direction to a point A 
may be obtained at once on the photograph-board by joining 
the station S, Fig. 73, Plate XL, to the projection a of the pic- 
tured point on the horizon line. 

If required in arc measure, the distance Pa is transferred 
to the principal line=PG, D is joined to G and produced to 
intersect the scale of degrees and minutes BC, where the gradu- 
ation mark K indicates the value of the azimuthal angle in arc 
measure. 

When many such angles are to be measured, the horizontal 
scales TY and OZ, Fig. 68, Plate XXXV, may be divided into 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 155 

degrees and minutes by means of a table of tangents, using the 
focal length SP as radius. 

' The altitude is the vertical angle at S, Fig. 73, Plate XL, 
of the right-angle triangle, having for sides Sa and aa. To 
construct it, take DF equal to Sa, draw FE parallel and equal 
to aa, join D and E and produce DE to the scale (BC) of degrees 
and minutes. 

This construction will be facilitated by the lines previously 
drawn on the photograph-board. With a pair of dividers take 
the distance (abscissa) from a to the principal line, carry it from P, 
Fig. 68, Plate XXXV, in the direction PD', and from the point 
so obtained take the distance to the arc ML, measuring it in 
the direction of the radials marked on the board, which will be 
the distance PF. Then with the dividers carry aa to FE, which 
is that one of the series If JV of parallel lines, Fig. 68, Plate XXXV, 
which corresponds to the point F. The construction may now 
be completed in the manner already explained. 

A protractor may be constructed to measure these angles 
directly. It consists of a transparent plate on which lines are 
drawn parallel to the principal line containing points of the 
same azimuth and curves containing points of the same altitude. 

The azimuthal lines are found by plotting the angles in 5 
and drawing parallels to the principal line SS' through the points 
of intersection with the horizon line. 

If we take the horizon and principal lines as axes of coor- 
dinates and .denote the altitude of a point pictured as a by h, 
the equation of the curve of altitude h may be written 

This also is the equation of a hyperbola of which the prin- 
cipal and horizon lines are the transverse and conjugate axes 
and of which the principal point is the center. 

One of the hyperbola's branches represents the points above 
the horizon and the other branch those of equal altitude below 
the horizon. 



156 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The asymptotes are lines intersecting each other at the prin- 
cipal point and making angles equal to h with the horizon line. 
This hyperbola is the intersection by the picture plane of the 
cone of visual rays forming the angle h with the horizon. 

These hyperbolic curves of equal altitude may be obtained by 
computation, using the preceding formula and substituting 
different values for h, or they may be obtained graphically by 
plotting a series of points for each curve, reversing the construc- 
tion given above for finding the altitude of the pictured point a. 
Fig. 73, Plate XL. The angular distance between the lines 
representing points of equal azimuths or those of the same 
altitude depends on the degree of precision aimed at. 

The complete protractor is shown in Fig. 74, Plate XL. It 
may be made in the same manner as mentioned for the per- 
spectometer by drawing it on paper on a large scale, reducing 
it by photography, and finally making a transparency by bleach- 
ing the negative in bichloride of mercury. 

D. Method of V. Legros for Locating the Horizon Line of a 
Vertically Exposed Plate. 

Commandant Legros recommends the use of these hyper- 
bolic curves for the location of the horizon line of a vertically 
exposed plate. 

When the camera with the photographic plate adjusted in 
vertical plane is rotated horizontally, the plate remaining ver- 
tical, any point a, Fig. 74, Plate XL, will describe a hyperbola 
a a' in the picture plane (on the ground -glass plate). The 
nearer a approaches the horizon hne the smaller the curvature 
of its hyperbolic trace on the ground-glass plate will become, 
and that point, a°, which traverses the ground-glass plate in 
a straight line, HH', will have the same elevation as the second 
nodal point of the camera-lens — its angle of elevation will be= -hO, 
or HH' will be the horizon line of the plate. 

To locate the horizon line experimentally in this way the 



GRAPHICAL ICONOMETRICAL PLOTTING METHODS. 1 57 

ground glass is best provided with a series of equidistant hori- 
zontal and vertical lij;ies, after the manner of Dr. Le Bon's ground- 
glass plates. 

E. Pro}. S. Finsterwalder's Method for the Iconometric Plotting 
of Horizontal Contours. 

Prof. Finsterwalder's method for plotting horizontal contours 
is well adapted for the development of the terrene forms of a 
moderately rolling country and it is based upon the following 
consideration : 

The pictured outline of a terrene form may be regarded as 
the trace of the terrene surface in a plane (picture plane) ver- 
tical to the plotting or ground plane. 

The camera stations should "be specially selected with refer- 
ence to the use of this method with a view toward obtaining 
pictures with a sufficient number of such outlines, or silhouettes, 
of the terrene forms to enable the iconometric draughtsman to 
give a good definition of the relief of the terrene to be plotted. 
These terrene-form silhouettes may be regarded as falling 
within vertical planes and the rays drawn from the point of 
view to the pictured points of the silhouette will form a cone, 
with apex in the second nodal point of the lens (or point of view), 
its base being formed by the pictured outline (silhouette) of the 
terrene. A horizontal plane containing a contour A will inter- 
sect such a cone of rays in a curve B, the latter touching A in 
one point. 

If we designate by h the difference in elevation between 
the station (whence the picture was obtained) and the hori- 
zontal contour ^, by /? the vertical angle of each radial or visual 
ray drawn to each point of the silhouette, then the curve B may 
be plotted on the working-sheet by laying off, upon a few rays, 
from the plotted station to points of the pictured outline the 
corresponding distances 

h cot /?, 



158 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

and the points thus located on the radials drawn from the sta- 
tion point, if connected by a continuous line, will represent the 
curve B, plotted in horizontal plan. 

The direction of the silhouetted outline is now plotted on 
the plan, and where it bisects this curve B wiU be a point of 
the contour A. As we, naturally, would draw not only one 
curve B but a series of them corresponding to several horizontal 
planes, passing through a series of contours A of various ele- 
vations, the construction may be simplified, inasmuch as the 
curves B — being the lines of intersection of the same cone of 
rays with a series of parallel planes containing the horizontal 
contours — will all be similar in shape, their corresponding points 
having the same relative positions with reference to the plotted 
station, and the value h cot /? need only be determined for one 
point of the remaining curves B if one curve B had been drawn;, 
the others will be parallel to it. 



CHAPTER VII. 

CAMERA-LENSES. 

The general theory and laws of optics as applied to lenses 
are the same whether the latter are to be mounted in telescopes 
or in photographic cameras. The camera may even be regarded 
as an incomplete telescope, lacking only a suitable eyepiece to 
convert it into a telescope. 

Still, photographic lenses are to fulfill requirements differing 
widely from those of telescopes, the main difference being in 
the field commanded by either. As only the central part of a 
telescopic lens is utilized for observing, comprising a field of 
but a few degrees, spherical and chromatic aberration do not 
affect the latter. Phototopographic lenses, however, should 
command as wide an angle as possible (over 60°) and still pro- 
duce geometrically true perspectives without distortion, with a 
sharp definition, a uniformly bright illumination for the entire 
plane surface of the sensitive plate, and with a great depth of 
focus. 

Rapidity in the action of the camera-lens being desirable,, 
but not of essential importance for surveying purposes, the 
quality of the lens will in a great measure determine the value 
of the photogrammeter or photographic surveying camera. 

A. The Refractive Index. 

With reference to Fig. 75, Plate XLI, we designate hy AB 
the refractive surface, by SI the incident ray, by IP the refracted 
ray, by CCi the perpendicular to the refractive surface in the 
point where the incident ray SI enters the second medium, by a 

IS9 



l6o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the angle included between the perpendicular CCi and the inci- 
dent ray SI, and by /? the angle of refraction. 

75 being equal to IP = r, the ratio of the sines of the angles a 
and j9 may be expressed by the ratio of the lines DS and EP, 
or, in other words, 

For all angles a, larger and smaller than the one indicated 
in Fig. 75, Plate XLI, the ratio between DS and EP will be 
.the same for the same two substances. 

This constant ratio is termed the " refractive index " of the 
two substances that are separated by the refractive surface AB. 
The incident ray, the refracted ray, and the emergent ray (coming 
from the same source) are all in one plane. 

When speaking of the refractive index of any one medium 
in optics, it is always to be understood that the incident ray 
has passed through air (or space). Thus we have, approxi- 
mately, if the refractive index for air or space be assumed as 
unity, the refractive index for 

Water, about 1.3 

For crown glass, about 1.5 

For flint glass 1.6 to 1.9 

For diamond, about 2.4, etc. 

This means, for instance, that for any angle a (for any 
incident ray SI) the vertical DS is 1.5 times as long as PE if 
the ray passes through air and is refracted by crown glass. 

B. Refraction of Light-rays. 

The preceding consideration enables us to find the means 
for changing the course of light-rays by refracting them to any 
amount desirable. 

With reference to Fig. 76, Plate XLI, we have AB and 
^iBi= refracting surfaces of a piece of plate glass. 

The incident ray SI arrives at the surface AB under an angle 
a with the perpendicular IC (perpendicular to AB in 7). Glass 
being denser than air the ray will be refracted toward the per- 



THE OPTICAL LENS. l6r 

pendicular IC, continuing in a straight line IE as long as it passes 
through this second medium (glass) ; arriving at E it passes from 
the denser medium into air and at E it will be refracted away 
from the perpendicular ECi (under an angle a) and continue 
in the direction EP, parallel to the incident ray SI. 

By changing the direction or position of one or of both sur- 
faces of the denser medium (glass) the final direction of EP 
may be given any course, since the equation 

sin a ... 

-: — ^ = « = refractive mdex 
sm p 

must always be fulfilled. 

It becomes plain that the change in the direction of EP from SI 
will increase directly with the angle included between the twO' 
refractive surfaces AB and AiBi. 

In Fig. 77, Plate XLI, this change in direction is shovm for 
three different glass prisms shaped in such a way that their 
refractive angles not only decrease from A toward B, but have 
been given such values that the three rays emanating from a 
certain luminous point S, after refraction, converge to one 
point P. 

A point P, where several converging rays (originally ema- 
nating from a point 5 in space) intersect one another, is termed 
an " image point." 

C. The Optical Lens. 

If the directions not only of three. Fig. 77, Plate XLI, but 
of an infinite n\uiiber of light-rays emanating from a luminous 
point 5 are to be so changed that all will converge into a point P, 
we will have to superimpose an infinite number of prisms one 
upon the other. The heights of these prisms will have to be 
made infinitesimally small and the refractive angles of two neigh- 
boring ones will differ by an infinitesimal small amount. 

This means that the broken lines AB and ABi, Fig. 77, 



l62 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Plate XLI, will become curves, and a piece of glass with its 
two faces shaped in such a manner that all light pencils ema- 
nating from the same point 5 will converge to meet in its image 
point P is termed an optical lens. 

Evidently such a lens is a body formed by rotating the fig- 
ure ABB I, composed of an infinite number of prism sections, 
about the line BBi as axis. This axis of rotation is termed 
the optical axis of the lens, and the latter may be considered 
as composed of concentric zones or rings with spherically shaped 
outer surfaces. The question now arises what form should 
be given the figure ABBi to obtain a lens that will produce optical 
images of luminous points. 

Opticians can produce in the manufacture of lenses only 
spherical surfaces with any degree of precision; therefore all 
optical lenses are inclosed by spherical surfaces. Still, spherical 
lenses produce well-defined and sharp images of luminous points 
only within certain limits, limits between which the spherical 
surfp,ce approaches very closely that ideal shape which is best 
adapted for the purpose in view, but which cannot be manu- 
factured owing to mechanical difficulties encountered in the 
grinding or cutting process of the lens. In our superficial treat- 
ment of the laws of optics — considered inasmuch as they apply 
to phototopography only — we shall assume that the spherical 
lenses are optically perfect and of a small thickness. 

The deduction of the optical laws governing the action of 
lenses of various shapes would require complicated computa- 
tions; still, at least a general consideration of certain optical 
laws and facts should not be omitted in this treatise on photo- 
topography, in order to better elucidate the formation of the 
optical images and to determine such elements of the photo- 
graphic lens as will be needed in iconometric plotting. 

Generally speaking, we meet with so-called simple lenses 
and with combinations or sets of lenses in photography. The 
symmetrical combinations are preferable for topographic sur- 
veying purposes, as they, command a wider field or larger view 



OPTICAL DISTORTION. 1 63 

angle and as they are less affected with distortion and aberration 
than is generally the case with the simple or single lenses. 



D. Optical Distortion. 

So-called " spherical aberration " is more commonly pro- 
duced by those light rays which pass through the marginal zone 
of the lens, as this part of the lens is less perfect than the cen- 
tral part. Spherical aberration may be reduced by decreasing the 
effective diameter of the lens which is generally done by insert- 
ing a so-called " diaphragm " between the lenses forming the 
combination, or by a reduction of the curvature of the faces of 
the lens. 

Lenses corrected for spherical aberration are known as 
aplcinatic lenses. 

In so-called " chromatic aberration " the different color 
rays which compose the white light are unevenly refracted, and 
colored, ill-defined images are the result. 

Lenses corrected for chromatic aberration are termed achro- 
matic lenses. 

Probably the greatest source of error introduced into photog- 
raphy is due to distortion of the image when using an inferior 
lens. It is caused primarily by the greater refraction — in the 
direction toward the optical axis — of those light-rays which pass 
through the marginal or border zones of the lens. When the 
image on the ground glass of a test-screen of the form shown 
in Fig. 78, Plate XLII, assumes the form indicated in Fig. 79, 
Plate XLII, so-called " pin-cushion distortion " has taken place, 
whereas an image of the fonn. shown in Fig. 80, Plate XLII, 
is produced by " barrel-shape distortion." A lens affected by 
either is vmfit for phototopographic purposes. 

Commandant Moessard has invented an ingenious little con- 
trivance — ^the tourniquet — by means of which the field or 
angle that is affected by distortion of the image may readily 
be determined experimentally. 



164 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Astigmatic distortion in an image is produced when well- 
defined images of the lateral points of an image may be obtained 
for two different positions of the ground-glass plate, and yet 
neither of these two images of the same points will represent 
the true shape of the original. Using a test-screen of the shape 
shown in Fig. 81, Plate XLII, radial distortion will be shown, 
Fig. 82, Plate XLII, for one position of the f ocusing-glass ; 
the distortion will be in directions radiating from the center 
of the ground-glass plate. In the other position of the focusing- 
glass tangential distortion will be observed, Fig. 83, Plate 
XLII; the distortions will appear in directions at right angles 
to the directions radiating from the center of the ground-glass 
plate. Both radial and tangential distortions increase from the 
center toward the extraaxial" zones of the lens. 

Lenses corrected for astigmatic distortion are termed anastig- 
matic lenses. 

The distortion shown in Fig. 84, Plate XLIII, of the image 
of the test-screen, Fig. 81, Plate XLII, is due to imperfect regis- 
tering of two lenses composing a double lens; the component 
lenses are not " centered." 

The Zeiss anastigmatic lens has a perfectly flat field. That 
is to say, if the ground glass has been focused for the sharp 
definition of a central point, extraaxial points will also be well 
defined on the focusing-plate. 

Nearly all the older lens types were characterized by more 
or less curvature of the field, which means the' focal length when 
focusing for a central point would be longer than when focusing 
for sharp definitions of a marginal point shown on the image 
plate. 



NODAL POINTS AND NODAL PLANES OF A LENS. 165 



E. Nodal Points and Nodal Planes of a Lens. 

Formerly the thickness of a lens was disregarded when inves- 
tigating its action upon Ught-rays passing through it and it was 
generally assumed that the central rays — those passing through 
the so-called " optical center " of a lens — suffered no change 
of direction. 

Lenses are generally regarded as being bounded by two 
spherical surfaces. If both sides are convex (the lens is thicker 
in the center than at the edge) it is termed a biconvex or 
positive lens, Fig. 85, Plate XLIII. 

If the spherical surfaces are concave on both sides of the 
lens (its center is thumer than its edge) it will be a biconcave 
or a negative lens, Fig. 89, Plate XLV. 

Fig. 86, Plate XLIII, represents the cross-section of a con- 
cave-convex or a periscopic convex lens, the convex surface 
having a shorter radius than the concave surface, the lens 
being thicker at its center than at its margin. When the con- 
cave surface has the shorter radius the lens would be called 
convex-concave. The principal elements of a lens (Figs. 85 and 
86, Plate XLIII) are: 

First. The geometrical centers; they are the centers C 
and Ci of the spherical surfaces forming the faces of the 
lens. 

The hne passing through C and Ci is termed the principal 
axis of the lens. 

Second. The vertices A and B of the lens are the inter- 
sections of the principal axis with the two spherical lens 
surfaces. 

Third. The thickness AB of the lens is the distance between 

the vertices of the lens. 
A lens is centered " when the plane PP, passing through the 
circumference of the -lens — passing through the circular line of 



1 66 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

intersection of the two lens surfaces — is intersected by the prin- 
cipal axis at right angles. 

A lens-combination is centered when the planes PPi of the 
individual lenses are parallel and if they are intersected by the 
principal axis at right angles. 

The foci of the separate lenses should also fall upon the 
principal axis or the images of the discs shown on the test-screen, 
Fig. 8i, Plate XLII, will show so-called " flare spots," some- 
what like those represented in Fig. 84, Plate XLIII. 

A large flare spot, or halo, in the center of an image or 
picture may be produced by halation, caused by light-rays that 
have passed through the diaphragm aperture being reflected from 
the lens surfaces. 

There exist certain relationships between the curvature of a 
lens, the distance of a luminous point from the lens, and the 
distance of its image from the lens which we will now briefly 
consider. 

An incident ray SI, Fig. 85, Plate XLIII, will be refracted 
at / toward the radius R (=CoI), glass being a denser medium 
than air; it will continue through the lens in the direction IE, 
and the emergent ray EP will be parallel to the incident ray SI. 
The radii CI and CiE are also parallel. The point Co, where 
the refracted part IE of the light-ray intersects the optical axis 
of the lens, is known as the optical center of the lens and 
the following relation exists between its distances from the geo- 
metrical centers and the radii of the two lens surfaces: 

CCq _R 
CiCo Ri 

The triangles ICqC and ECqCi are similar. 

Every lens has two nodal points N and iVi, Figs. 85 and 
86, Plate XLIII, on the optical axis of the lens. The rays reach- 
ing the first nodal point N from luminous points S in space 
are parallel with the rays connecting the second nodal point Ni 
with the corresponding images P of the luminous points. 



NODAL POINTS AND NODAL PLANES OF A LENS. 167 

Hente a negative produced by an optical-lens system may 
be regarded as a central projection or as a perspective image 
(the center of which coincides with the second nodal point Ni), 
Fig. 88, Plate XLIV. 

The positions of the nodal points will be constant for all 
rays that make a small angle with the optical axis of the lens 
(for all rays passing through the small aperture of a diaphragm). 
The distances of the nodal points from the corresponding ver- 
tices of the lens are constants and their values are given by the 
equations (Fig. 85, Plate XLIII) 

^^^ n CCo' 
_BCo C^I^ 

where n is the refractive index from air into glass, or w=|-. 

As the distances of the nodal points from the optical center 

(CfliV and CqNi) will be small — they may sometimes become 

. , . CN , CiNi 

inappreciable or=o — we may omit the factors ~p;r=r and -pry^ 

CCo ClCo 

from the equations (when CoN=o and CoNi=o the factors 

;p:r=~ and „ „ will become =1), hence 
CCo CiCo 



BNi = 



n 

BCo 
n 



A close approximation to the distance between the nodal 
points will be 



n — I 
iViVi = AB. 



l68 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The planes and the points of a lens system are numbered 
in the sense of the direction of the incident rays. With refer- 
ence to Fig. 87, Plate XLIV, the light is supposed to be coming 
from the left, hence 

iy= first nodal point; 
i<'= first principal focus; 
i<'G= first focal plane; 
iJ„iVi<C= first nodal plane, whereas 
iVi= second nodal point; 
^1= second principal focus, etc. 

Lengths are considered plus, or positive, if they extend in 
the sense of the direction of the incident rays, and minus, or 
negative, if they extend in the opposite direction. With refer- 
ence to Fig. 87, Plate XLIV, where the light comes from the 
left, we have 

FN=-f and FiiVi=-f/i. 

The nodal planes (passing through the nodal points and 
intersecting the principal axis at right angles) coincide with the 
principal planes if the extreme or outer medium of the optical 
lens system is the same, which is the case in photography where 
air surrounds the lens. 



F. Principal Foci and Focal Planes of a Lens. 

The principal foci F and Fi of a biconvex or positive 
lens. Fig. 87, Plate XLIV, are two points on the optical axis — 
one on either side of the, lens — where those incident rays con- 
verge which arrive at the refractive lens surface in a course 
parallel to the optical axis. 

In Fig. 87, Plate XLIV, the ray S'P^, coming from a lumin- 
ous point S' at infinite distance from the lens, traverses a path 
parallel in its course to the optical axis FFi of the lens, and 



THE FOCAL LENGTH OF A LENS. 169 

after refraction converges to the point .Fj; while a similar ray 
PE", coming from the opposite direction in a course parallel 
with the optical axis, converges to F. 

The planes FG and Fid, passing through the foci F and i^i 
and intersecting the optical axis FFi at right angles, are termed 
focal planes. 



G. The Focal Length of a Lens. 

FN and FiNi, Fig. 87, Plate XLIV, are termed focal 
lengths and they are generally designated by the letter /. The 
value of the focal length is expressed by the equation 



A=-/=- ^' 



in-i)(Ri-R+'~-AB) 



When this value for }i becomes positive it is an indication 
that the incident rays, when coming from infinite distance (parallel 
with the optical axis), are refracted to converge to the principal 
focus of the lens. 

A negative value for /i would indicate that the rays entering 
the lens in a course parallel to its optical axis diverge from the 
principal focus. 

For thin lenses (the distance between A and B, Fig. 85, 
Plate XLIII, is very small in comparison with the lengths of 
the radii R and Ri and it may be assumed =6) the formiUa 
for the focal length would read 

■ RRi 

'-{n-iXRi-R)' °^ 



170 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

After substitution of f for «, the approximate value for the 
refractive index of glass, the approximate value of the focal 
length for thin lenses would be 

III 

H. The Biconvex or Positive Lens. 

The image of a point at infinite distance from the biconvex 
lens is on the opposite side of the lens and falls together with 
its principal focus. 

When the distant luminous point approaches the lens the 
image will recede, at first slowly, but more rapidly the nearer 
the luminous point advances toward the lens, and by the time 
the original point will have reached a distance from the lens 
equal to its double focal length, its image will have moved to a 
point beyond the lens, also at a distance of the double focal length. 
When the luminous point continues to approach the lens within 
the double focal distance range, its image moves faster and 
faster beyond the double focal distance on the opposite side of 
the lens, and when the luminous point finally falls together 
with the first focus (F) the image will disappear at infinite dis- 
tance. 

The relation which exists between the position of a luminous 
point (5) and that of its image (P) may be briefly expressed in 
the following equation: 

III 

where, with reference to Fig. 87, Plate XLIV, 

/ = principal focal length of lens; 

0= distance (SH=SiN) between the lens (its first nodal plane) 

and the luminous point (5) ; 
J=distance (iViPi) between the lens (its second nodal plane) 

and the image plane PPi. 



CONJUGATE 


FOCI AND CONJUGATE 


PLANES. 


From the above equation we deduce 






}-b 






* = /-• 






^_a.b 





171 



' a+b' 

With these simple formulae any question concerning the 
distance between the image and the lens (its focal planes) may 
be solved. 

We may have, for example, a lens of 15 cm. focal length 
and the object (the luminous point S, Fig. 87, Plate XLIV) 
may be 50 cm. away from the lens. It is desired to find the 
distance between the nodal and image planes (the distance PiNi). 

For/= —15 and 0= —50 we find, 

from b=—, — , 
}-a' 

I. Conjugate Foci and Conjugate Planes. 

The image Pu, Fig. 87, Plate XLIV, of a luminous point U, 
situated on the optical axis of a biconvex or positive lens, will 
also be on the optical axis of the lens, but on the opposite side 
of the latter. The incident ray UIu, emanating from the axial 
point U, will be refracted beyond lu, and its course may be found 
by drawing a ray FH', parallel with UIu, through the first prin- 
cipal focus F, which ray, after having traversed the lens, will 
emerge in a direction H'Gu, parallel to the direction of the 
optical axis. This fictitious or auxiliary ray H'Gu intersects 



172 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the second focal plane Fid at Gu, and if we draw a line from 
Ku .through. Gu it will represent the path of the emergent ray 
originally emanating from U, and the intersection Pu of i^jFi 
with KuGu will locate the image Pu of the luminous axial point U. 

The point U and its image Pu (when axial points) are termed 
" conjugate foci." 

The planes TuPu and TU, both vertical to the optical axis 
and passing through the conjugate foci U and Py, are termed 
conjugate plapes. 

K. To Find the Image of any Luminous Point for the 
Biconvex Lens. 

The image Tu, Fig. 87, Plate XLIV, of any luminous point T 
in the conjugate plane UT may be found by locating the point 
of intersection Tu of the emergent rays of: 

First, an incident ray TI^^M, drawn parallel to the axis FFi ; 

Second, an incident ray TN, drawn through the first nodal 
point N, and. 

Third, an incident ray TF, drawn through the first principal 
focus F. 

If the conjugate plane TuPu had already been located, 
the drawing of the third ray TF would suffice to locate the image 
Tu of T, as it is in the intersection of TuPu with the emergent 
ray H'Tu of the incident ray TF. 

Knowing how to locate the image P of any luminous point 
5 or r for the biconvex or positive lens we can now locate the 
images of lines (as these may be regarded as a series of an infinite 
number of points), and also of surfaces (being composed of an 
infinite number of lines), provided the objects are not too far 
away from the optical axis of the lens. 

The image P of any luminous point S, Fig. 87, Plate XLIV, 
not on the optical axis is found graphically by locating the point 
of intersection (after refraction) of the three following specific 
rays emanating from the point S: 



THE BICONCAVE OR NEGATIVE LENS. 1 73 

First. Draw a ray from 5 through the first principal focus F 
and produce it to the intersection with the first nodal 
plane at K, whence it continues in a direction parallel 
to the principal axis (having passed through the first 
principal focus). 
Second. Draw a ray from 5 parallel with the optical axis 
of the lens to its intersection Hi with the second nodal 
plane, whence it converges to the second principal focus J^i 
(having arrived at the lens in a direction parallel to its 
optical axis) and produce it to its intersection in P with PK. 
Third. Draw a ray from 5 to the first nodal point N; it 
will pass through the second nodal point Ni, and it will 
emerge at E' in a direction E'P parallel to the directipn 
of the incident ray SPN. 
The image of any point 5 situated in a plane SSi perpen- 
dicular to the optical axis will fall within the conjugate plane PiP. 
In Fig. 90, Plate XLV, where similar points are designated 
by the saine letters correspondingly used in Figs. 87 and 89, 
it has been shown how the image P'PiP of a line SSiS' may 
be found if the incident rays are refracted by a biconvex lens. 
The preceding definitions and formulas are applicable only 
to fight-rays which make small angles with the optical axis (for 
lenses with diaphragm stops), and they serve to illustrate and 
explain the formation of images. 

More rigid (and consequently more complicated) formulae 
would have to be applied to ascertain the best shape of lenses for 
special purposes. 

L. The Biconcave or Negative Lens, 

The biconcave or negative lens produces upright virtual 
images of originals which are beyond the principal plane, whereas 
the biconvex or positive lens,^ as has been shown in Fig. 90, 
Plate XLV, produces inverted real images of objects. 

If the object UT, Fig. 91, Plate XL VI, is situated between 



174 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

a positive lens and its principal focal plane FG, its rays will 
produce a virtual upright image PyTu. 

Incident rays that are parallel to the optical axis of a posi- 
tive lens will converge to the principal focus of the lens, but 
with the negative lens such rays will, after refraction, diverge 
in directions coming from the principal focus. 

It will readily be seen, with reference to Fig. 89, Plate XLV, 
that the image PPi of an object SS\ is upright and virtual. 
The paths of the light-rays are given in. full lines and similar 
points are given the corresponding designation as in the pre- 
ceding figures for the biconvex lens. 



M. To Find the Image of a Luminous Point for a 
Biconcave Lens. 

To find the image P of a luminous point 5 beyond the prin- 
cipal focal plane -FiGi, Fig. 89, Plate XLV, three incident rays 
are drawn: 

5/1, parallel to the optical axis; 

5/2) through the principal point F; 

SI3, through the first nodal point N. 

The intersection of the backward prolongation of the three 
corresponding emergent rays, PEi, PE2, and PE3, will locate 
the image P of the luminous point 5. These two points, P 
and S, are termed conjugate points, the same as mentioned for 
the positive lens. 

N. Lens Combinations. 

In Fig. 92, Plate XL VI, a combination of a single positive 
and one negative lens is shown. The positive lens may have: 



jFiV=i?iiVi= focal length; 
FG and FiGi = focal planes; 
NM and iViiTp= nodal planes. 



LENS COMBINATIONS. 1 7 15 

The negative lens may have: 

FN' = Fi'Ni' = focal length ; 

F'G' and i*'i'Gi' = principal focal planes; 

N'K and iVi'Z,= nodal planes. 

To find the principal focal planes of the lens combination we 
proceed in the same way as with a single lens, bearing in mind 
that the incident ray of the second lens is now the emergent ray 
of the first lens. 

The line 57 represents an incident ray arriving at the positive 
lens in a direction parallel to the optical axis; it is produced or 
continued in its course until it reaches the second nodal plane 
of the positive lens, where it changes its direction to one bisecting 
the second principal focus of this lens. In this direction it is again 
produced until it reaches the first nodal plane, in K, of the nega- 
tive lens, whence it continues to the second nodal plane Ni'L 
to the point L, the hne KL being parallel to the optical axis. 

Now we draw through the first principal focus F' of the nega- 
tive lens the auxihary ray YF' parallel with HFi. YX, drawn 
parallel to the optical axis, intersects the second focal plane F/d' 
of the negative lens in X, and XL will be the direction of the final 
emergent ray; its intersection Fi" with the optical axis is the second 
principal focus of the lens combination. 

This point, Fi", may be checked with a second incident ray 
Sill, and Fi'Gi" will be the second principal focal plane of this 
lens combination. 

In a similar manner t-^o incident rays PF and Pili' arriving 
at the negative lens from the other side of the combination, under 
a direction parallel to the optical axis, will locate the first prin- 
cipal focal plane F"G" of this lens combination. 

The first nodal plane of this combination {=N"a^) is located 
by determining the intersections a and ^ of the original incident 
rays PF and Pili with the final emergent rays F"T and FTi 
respectively. 

The second nodal plane of this lens combination is fixed by 



176 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the intersections of the original incident rays 57 and SJi with 
the final emergent rays Fi'X and Fi"Xi respectively. 



0. Diaphragms or Lens Stops. 

It had already been mentioned incidentally that diaphragms 
are used to reduce the aberrations of Ught-rays which arrive at 
the marginal zones of a lens, by excluding them from action 
upon the photographic plate. 

By selecting a sufficiently small aperture in the diaphragm 
all rays may be excluded from reaching the interior of the camera, 
which make an angle' with the optical axis larger than the angle 
controlling the limit of the central field of the lens that may be 
regarded free from distortion. 

This would comprise that effective circular disc of a lens for 
which the preceding optical laws and rules have been given, as 
the conditions are different for the extra-axial zones of a lens, 
and those rules are not appUcable to the latter. The laws given 
in the preceding pages become less and less true the nearer the 
■outer margin of the lens is approached. 

By the insertion of a diaphragm stop, a more or less great 
amount of Ught will be excluded from action upon the sensitized 
film of the photographic plate, and the smaller the aperture in 
the diaphragm the longer the exposure will have to be made in 
order to reduce a given amount of silver in the sensitized film. 

Generally speaking, the quantity of light admitted into the 
camera will be proportional to the square of the diameter of the 
diaphragm aperture. 

P. Rapidity of a Lens. 

Lenses with comparatively short focal lengths will produce 
brighter images than such with long focal lengths, the brightness 
of the image being inversely proportional to the square of the 
-focal length. The more light is allowed to enter the camera the 



DISTORTION PRODUCED BY DIAPHRAGMS^ 1 77 

quicker the reduction, of the chemical compounds of the sensi- 
tized film will take place; the rapidity of a lens depends in a great 
measure upon the quantity of light which the lens will suffer to 
reach the plate. 

Small apertures necessarily will permit more light to reach 
the central part of the plate than reaches its extra-axial parts, 
and photographs obtained through small diaphragm apertures 
often are darker and lack good definition on the edges. 

If the sensitive plate could be made less sensitive to the action 
of the light in its central part than it is on the edges this draw- 
back would be overcome, in a great measure at least; practically, 
however, the sensitized coating is of a uniform character. 

If d represents the diameter of the diaphragm aperture, and 
if / represents the focal length of the lens, then the rapidity of the 
lens (or the brightness of the image produced with that lens) 
will be proportional to the fraction 



& 



Q. Length of Exposure. 

Generally speaking, the length of exposure that should be given 
a plate is inversely proportional to the rapidity of the lens, hence 
proportional to the fraction 



&■ 



R, Distortion Produced by Diaphragms. 

When a diaphragm is placed in front of a positive lens so-called 
" barrel-shape " distortion (Fig. 80, Plate XLII) frequently 
ensues in the border regions of the image, and a diaphragm placed 
behind the lens is apt to produce so-called "pincushion" dis- 
tortion (Fig. 79, Plate XLII). It has been sought to compensate 



178 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

these distorting effects by using two lenses and inserting the 
diaphragm between them. 



S. Chromatic Aberration of Light-rays. 

The researches of Dolland, made with a view to reduce or 
overcome chromatic aberration of telescopic lenses, led to the 
combination of different glass compounds in the same lens, or 
better, to the combination of two or more lenses each of which 
was made of a glass mixture of different but well-known qualities 
regarding both dispersoin and refraction. He was successful in 
thus eliminating from the old-style lenses the greater part of the 
chromatic aberration which shows itself in the more or less 
pronounced appearance of colors on the borders of an image with 
a simultaneous indistinctness of outline. 

The improvement in this respect of all modem photographic 
lenses is principally due to the results obtained in the optical 
factory of Zeiss in Jena, where extensive experimental researches 
were made by Dr. Schott by direction of Prof. Abbe. By a 
judicious selection and combination of the glasses obtainable 
from the works at Jena, opticians can now produce lenses more 
fully answering the different requirements for the various uses 
to which photography may be applied than has heretofore been 
deemed possible. 

Still, so-called achromatic photographic lenses cannot yet be 
made free from all chromatic aberration, as no two kinds of glass 
have yet been compounded to precisely counteract or neutralize 
the refractive errors inherent to each. 

That amount of aberration with which so-called achromatic 
lenses still remain affected is known as secondary chromatic 
aberration. It has been reduced to such a degree that its dis- 
turbing effect in achromatic telescopes — where small angles of 
the field are only used — disappears altogether, but in lens com- 
binations for photographic work, and particularly in phototopog- 
raphy (where large. field angles are used), this permanent defect 



CHROMATIC ABERRATION OF LIGHT-RAYS. 1 79 

is still seriously felt, particularly when short focal lengths of the 
camera-lenses become desirable. 

Achromatic photographic double lenses are composed of 
two or three lenses each, the glass of the single lenses being care- 
fully selected with a view towards overcoming the chromatic 
aberration of the light-rays as much as possible. 

So-called white or colorless Ught is composed of a series of 
colored light-rays intermingled in such a way that their joint 
effect is that of colorless light. The main characteristic of these 
color rays with reference to our subject is " each of the different 
light-rays that form the component parts of white light has a 
different refractive index for the same medium;" or, light- 
rays of different colors will be refracted under different angles 
for the same refractive niedium. Red rays, for example, are 
less refracted than yellow rays, and these again are less refracted 
than the blue and violet light-rays for the same refracting medium. 

If a pencil of white Ught be intercepted by a glass prism 
it will become separated into its component color rays; each 
different color, representing a different wave-length, is differently- 
refracted by the prism, each color having its own special iridex 
of refraction. 

The prism separates a,nd disperses the different' color rays 
which compose the white light, and when the refracted rays 
are cast upon a white screen in a dark-room, the band of colors 
appearing on the screen is termed the spectrum of the particular 
light used. 

If a pencil of sunlight had been used in the experiment the 
solar spectrum would appear on the screen and the red rayg 
will be less refracted than the orange, the green, the blue, and 
the violet rays. 

We had seen (Fig. 77, Plate XLI) that a biconvex lens may 
be regarded as a series of concentric prismatic rings, one super- 
imposed upon the other, and it will be evident that a chroinatic 
lens wiU retract red rays less strongly than the orange, green, 
or violet rays. The lens will have a shorter focus for the violet 



l8o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

and blue rays than it has for the orange or red rays, and the focal 
length of such a lens will vary according to the particular color 
of the emergent ray (Fig. 93, Plate XL VII). 

As the retina of our eye is more sensitive to the yellow and 
green light rays than to those of another color, we will, whea 
focusing upon a landscape view, perceive the best definition 
when the focusing-plate falls together with the focal plane of the 
lens focus for the yellow or light-green light-rays. 

Generally speaking, however, the ordinary photographic 
dry-plate emulsion is less sensitive to yellow light and more 
sensitive to blue or violet light-rays, and in order to obtain a 
good negative, the plate should be exposed in the focal plane 
of the blue or violet Ught-rays instead of being exposed in the 
focal plane of the yellow rays, as it would be if exposed in the 
position as determined with the ground glass for the best ocular 
definition. 

The distance D, Fig. 93, Plate XLVII, between the focal 
planes of the yellow rays (active rays optically or visually) and 
the blue-violet (or chemically active) rays is termed the chro- 
matic aberration or variation of the rays. Lenses affected with 
chromatic aberration have a chemical focus, differing more or 
less from the optical or visual focus, according to the more or 
less great amount of chromatic aberration by which the lens may 
be affected. 

Compositions of glass of different indices of refraction will 
separate white light into spectra of different lengths. Lenses 
made of glass having a small refractive index will show a small 
difference in the focal variation D, Fig. 93, Plate XLVII (between 
the focal lengths), of the red and violet light-rays. Such glass 
is generally known as crown glass and its index of refraction is 
from 1.5 to 1.6. 

Lenses made of a composition of glass having a strong refrac- 
tive power will show a greater focal variation. Such glass is 
known as flint glass and its index of refraction is from 1.6 to 1.9. 

Any crown-glass positive lens of a given focal length may 



CHROMATIC ABERRATION OF LIGHT-RAYS. l8l 

be matched with a flint-glass negative lens of a larger focal length 
(Fig. 92) that maybe of such a refractive power to bring the chem- 
ical focus of the combined pair almost into coincidence with the 
optical or visual focus without annulling the entire refractive 
power of the positive lens. A lens combination of this character 
which still retains the characteristics of the positive lens (posi- 
tive focal length £ind real image of a distant object) and which 
is almost free from chromatic aberration is termed an achro- 
matic lens combination. 



CHAPTER VIII. 

PHOTOGRAMMETERS OR PHOTOGRAMMETRIC 

INSTRUMENTS. 

Photographic surveying instruments have already under- 
gone many changes and various patterns are in use in differ- 
ent localities. Until quite recently photogrammeters were not 
procurable in open market. Nearly every observer who made 
use of the photographic method for topographic surveys had 
an apparatus constructed especially for his particular needs and 
according to his personal ideas. Thus we find : 

First. The ordinary photographic field camera (with bellows 
extension) converted into a surveying camera by a few 
simple additions and mechanical modifications. 
Second. A specially constructed surveying camera with a 
constant focal length and special devices for leveling (to 
bring the sensitive film into vertical plane). 
Third. A surveying camera combined with some geodetic sur- 
veying instrument (with a surveyor's compass, a transit, 
or with a plane table). Such combination may be per- 
manent, or it may be effected in such a way that the camera 
is detachable and both may be used independently and 
separately. 
The practical value of a photogrammeter depends largely 
upon the quahty and general uniformity of the lens or lens com- 
bination upon the rigidity of the component parts of the apparatus, 
its transportability, and upon the rapidity with which it may 
be adjusted and placed in position for use. 

182 



PHOTOGRAMMETRIC INSTRUMENTS. 183 

The principal lenses that have been employed for photo- 
topographic purposes are Dallmeyer's rapid rectiUnear, Stein- 
heil's aplanatic, Busch's pantoscopic, Gorz's double anastig- 
matic, Voigtiander's collinear, and more recently Zeiss's anastig- 
matic lens. 

The nodal points, the focal length, the arc of visibility, and 
the arc which is perfectly free from distortion of any kind should 
be known for every lens used for phototopographic purposes, 
and the manufacturers of lenses of good quality are best fitted 
to determine these values with great precision. 



General Requirements to be Fuifilxed by a Topographic 

Surveying Camera. 

A good surveying camera or photogrammeter for topographic 
work shoiild produce negatives that are geometrically true per- 
spectives. The elements of the latter sh'ould be known and the 
following. desiderata should be fulfilled: , 

First. The photographic plates should be adjustable in ver- 
tical plane. 

Second. The distance between nodal and image plane 
should be maintained unchanged for aU exposures. 

Third. This distance-^the constant focal length — must be 
known, or- will have to be determined for every instru- 
ment. 

Fourth. Means should be provided to trace the horizon line 
(line of intersection of the horizon plane with thp vertical 
photographic plate) upon every negative. 

Fifth. Means should be provided for locating the principal 
point (the point of intersection of the horizontal optical 
axis of the camera with the vertical sensitive plate) upon 
every negative. 

Sixth. A ready orientation of the photographs for icono- 
metric plotting should be possible; and we may add aa 



184 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Seventh. Enough characteristic stations (outside of the tri- 
angulation scheme) are to be occupied with the camera 
to give a full development of the terrene, which is to be 
mapped. 



I, Ordinary Cameras (with Extension Bellows) Converted into 
Surveying-cameras. 

These surveying-cameras have been constructed primarily for 
economical reasons and their use should not be extended beyond 
preUminary work or beyond surveys made for experimental study. 
For extensive use the results will not be sufficiently uniform and 
accurate. 

Such cameras are generally supported by three leveling-screws 
upon a tripod and they are provided with a circular level /, Fig. 95, 
Plate XL VIII, or with two cross-levels L, Fig. 94, Plate XL VII, 
for adjusting the sensitive plate into vertical plane. The dis- 
tance between nodal plane and photographic plate is made invari- 
able, generally by means of two metal rods R, as shown in Fig. 94, 
Plate XL VII (Werner's apparatus made by R. Lechner in 
Vienna, Austria). 

In Fig. 95, Plate XL VIII (apparatus of Dr. Vogel and Prof. 
Doergens, made by Stegeman in Berlin, Prussia), this has been 
accomplished by means of the clamp M. After the bellows^ 
have been extended sufficiently to establish the desired focal 
length, which may be read off on the vernier n, the screw M is 
securely clamped. The pinion K with rack movement zz serves 
to give the needed slow motion (when extending the bellows) 
to set the vernier n. 

Dr. G. Le Bon also used a modified field camera for his 
archaeological researches in India, which were carried on under 
the auspices of the French Ministry of Culture. 

The braces H in Fig. 95, Plate XL VIII, and R in Fig. 94,. 
Plate XL VII, give the plate receivers a vertical position upon 
the level extension boards. 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 185 

The short brass points m in Fig. 95, Plate XL VIII, locate 
the principal and horizon lines by their reproduction on the 
negatives. They are brought into actual contact with the plate 
(before exposure) by turning the buttons h, thus producing a 
sharp image of their outlines on the margins of the negatives. 

Fig. 96, Plate XLVIII, shows an arrangement for setting 
the four teeth (which locate the horizon line hhi and the prin- 
cipal line i;?;! on the negative) close against the sensitive film 
surface, which has been used a great deal in Germany. By 
turning the arms r the teeth may be brought into direct contact 
with the plate, and when the camera should be used to obtain 
pictures for their pictorial value only the teeth may be removed 
from the plate by turning the buttons a, b, c, and d back. 

The original Coast Survey Camera was provided with a 
device which would operate all four teeth together by turning 
but one button. 



II. Special Surveying-cameras with Constant Focal Lengths. 

Among the numerous patterns of this class of instruments 
that devised by Dr. Meydenbaur is probably the earliest form. 

A. Dr. Meydenbaur' s neiD small Magazine Camera. 

This instrument is represented in Figs. 97 and 98, Plate 
XLVIII. The camera weighs 750 grams and the plates are 
9X12 centimeters in size. The camera-box is mounted, by 
means of ball-and-socket joint, upon a vertical rod which is joined 
at the bottom to three short legs in such a way that the four 
pieces may be folded together to form a stout cane 0.85 m. long. 
The lower ends of the three legs of this tripod and the upper 
end of the supporting staflE are connected by twisted violin-strings. 
Tension may be given them by turning the ratchet-wheels indi- 
cated in Fig. 98, Plate XLVIII, thus producing a very light 
and yet rigid tripod. The plates contained in the magazine M 



1 86 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

are exposed successively by being pressed against a metal frame 
having marks indicating the principal and horizon lines. This 
frame is securely fastened at a constant distance from the lens, 
establishing the constant focal length of the camera. Each 
plate, after exposure, may be dropped into the leather pouch 
b, Fig. 98, Plate XL VIII, secured underneath the camera, by 
manipulating a button a'. The leather pouch, together with 
one dozen plates, weighs about 500 grams. Dr. Meydenbaur 
has used a pantoscopic lens, made by E. Busch in Rathenow, 
Prussia, which is said to produce geometrically correct per- 
spectives of an angular field of 105°- A circular level L serves 
to adjust the plates into vertical plane before exposure, using 
the ball-and-socket joint for this purpose. 



B. Capi. E. Deville's Surveying-camera (new Model). 

The following description of Capt. E. Deville's new survey- 
ing-camera is taken from Deville's " Photographic Surveying," 
second edition (1895). The camera is shown in Fig. 99, Plate 
XLIX, and Fig. 100, Plate L; Figs. loi and 102, Plate LI, 
represent sections of the instrument. 

The camera proper is a rectangular metal (aluminum) box 
AB, Figs. loi and 102, Plate LI, open at one end. It carries 
the lens L and two sets of cross-levels CC, which may be read 
through openings in the outer mahogany casing. The metal 
box is supported in the wooden casing by wooden blocks and 
by a wooden frame FFi held in position by two bolts DD. 

The plate-holder is made for single plates; it is inserted 
into the carrier EE, which may be moved forward and back- 
ward by means of the thumb-screw G. 

A folding shade, of wood, HH, Figs. loi and 102, Plate LI, 
hooked to the front of the camera, and diaphragms KK inside 
of the metal box, intercept all light-rays which do not contribute 
to the formation of the image on the photographic plate. 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 187 

The camera rests on a triangular base, Fig. 103, Plate LI, 
with foot-screws, in shape exactly like the base of the transit 
theodolite which is used in conjunction with Deville's survey- 
ing-camera, so that either instrument may be placed on the same 
tripod at any station. The camera may be set up on the tri- 
pod with either the long or the short side vertical. 

Both transit and tripod are carried by the surveyor, and one 
camera including one dozen plates (in the single holders) with- 
out a tripod are taken by one of the men who always accom- 
pany the surveyor as packers. The assistant has a second 
camera with plates and a separate tripod. 

The legs of these tripods when folded together are twenty 
inches long. They are placed in separate cases and one is carried 
under the box of the transit, to be carried on the back of the sur- 
veyor, and the other is attached to the sole-leather case of the 
camera in charge of the assistant surveyor. 

The lens of this camera is a Zeiss anastigmat. No. 3 of series V, 
141 mm. focal length, with a deep-orange color-screen in front. 

Having mounted the camera on the tripod, the plate-holder 
carrier E is moved back as far as it will go by turning the screw G, 
Figs. loi and 102, Plate LI; the plate-holder is inserted through 
the opening M, the slide is withdrawn, and the carrier moved 
forward by revolving the screw G until the plate is brought into 
contact with the back of the metal box AB. In order to secure 
a perfect contact, the carrier E has a certain amount of free 
motion. The camera should now be turned in the proper direc- 
tion; the field embraced by the plate is indicated by Hnes drawn 
on the outside of the mahogany casing. The camera is then 
carefully leveled, the exposure given, and (.he plate-holder is 
withdrawn by repeating the same operations in the inverse order 
as described for its insertion. 

For the sake of rigidity and to reduce the number of adjust- 
ments to be made in the field to a minimum, the levels CC have 
been fixed rigidly to the metal camera-box (without any device 
for subseqeunt adjustment). They are, however, very closely 



1 88 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

adjusted by the maker of the instrument. For this purpose he 
takes the metal camera-box out of the mahogany casing and 
deposits the same on a piece of plate glass which previously 
had been leveled like an artificial horizon. By filing down 
one end or the other of the levels' outer case, each bubble is 
brought as near to the middle of its tube as possible. The 
graduation on the latter is numbered continuously from end to 
end, as illustrated in Fig. 104, Plate LI. 

Each camera ]s accompanied by a piece of plate glass \ inch 
thick and 11 inches long, which can be inserted into the carrier 
in place of the plate-holder. That end of the glass plate which 
projects beyond the camera limits when it is thus inserted is 
coated on the back with a varnish composed of gum guaiacum 
dissolved in alcohol to which some lampblack had been added. 
This coating has very nearly the same refractive index as glass 
and is well adapted for precluding all reflections from the back 
of the glass plate. 

After the camera has been received from the maker the exact 
readings of the levels, when the back of the metal box — against 
which the photographic plate is pressed — is vertical, should 
be ascertained. To do this, the bolts P, Fig. 102, Plate LI, 
next to the opening M, are loosened and removed. Q may 
then slide backwards and be taken out. The piece of plate 
glass is now inserted in the carrier E, Figs. loi and 102, Plate LI, 
and pressed into contact with the metal box. The camera is 
placed on its tripod and leveled. Immediately in front and 
at the same height a transit (or a leveling instrument) T, Fig. 105, 
Plate LI, is set up, and after carefully adjusting it, a distant 
point P is selected on the same level with the transit and camera. 
The intersection of the cross-threads in the telescope is brought 
to coincide with P, and the telescope is clamped to the vertical 
circle. Turning it around in azimuth, the image of P, reflected 
by the plate glass, should appear in the intersection of the tele- 
scope's cross-threads. If it does, the face of the plate glass is 
vertical and the position of the bubble in the tube of the level, 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 189 

directed at right angles to the plate glass, is the correct one for 
adjusting the instrument in the future, , If it does not, the camera 
must be tilted forward or backward by means of the foot-screws 
until coincidence is established. The middle of the bubble 
of the level may or may not now be in the middle of the tube, 
but its position, whatever it is, will be the correct one for adjust- 
ing the camera in its subsequent use. The divisions of the 
graduation between which the bubble rests should therefore 
be ascertained and the middle reading be recorded, and when- 
ever the camera is to be leveled, it must be remembered that 
the middle of the bubble is to coincide with the recorded middle 
reading. 

This determination of the level-reading is to be made for 
the two positions of the camera in which it is used (horizontal 
and vertical position). 

1. Determination op the Focal JLength, the Horizon Line, and the 

Principal Line. 

The next step is to fix the position of the principal point 
on the photographic plate and to ascertain the length of the 
distance line. Select a station so that a series of distinct and 
well-defined distant points may be found on the horizon line 
as it is laid down by the maker of the camera. The selected 
view may comprise the distant shore line of a lake, a large build- 
ing or a row of buildings. Set up the tripod and adjust the transit. 
Find two points E and F, Fig. io6, Plate LII, on the horizon 
line (with a zenith distance of 90°) that both come within the 
field of the camera, when set horizontal, and fall near the two 
vertical edges of the plate. Measure the horizontal angle w 
between them. Find two other points G and H, also on the 
horizon line and such distance apart that both come within the 
field of the camera when the same is set up vertical. Now replace 
the transit by the camera in the horizontal position and turn 
it so that E and F will appear within the limits of the plate, level 



190 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

carefully, and expose the plate. Set the camera in the vertica. 
position and turn it in azimuth to take in G and H, level care- 
fully, and expose another plate. 

The . first plate, after development, shows the two points 
E and i^ on a line very nearly parallel to the edges AB and CD, 
Fig. 106, Plate LII, of the metal box. The principal point, of 
course, will be on this line, which is cut into the film, using a 
fine needle point and straight-edge for this purpose. 

The second plate, exposed in the vertical position of the 
camera, will give another horizon Hne GH, which may be trans- 
ferred to the first plate by means of the distances AK and CL 
to the comers of the metal box. This (principal) line is like- 
wise cut through the film of the photographic plate with a fine 
needle point and straight-edge. The principal point P will be 
at the intersection of these two horizon lines EF and GH. 

The length of the distance line (5P = /), or the focal length 
of the camera, may be computed from the horizontal angle w, 
included between SE and SF, together with the distances EP = a 
and PF=b. 

Let S, Fig. 107, Plate LII, be the second nodal point of the 
camera-lens, a and /? the angles ESP and PSF, when 

The lengths of a and b are measured directly on the plate. 
If we designate the focal length PS by / we have: 



a 
tana=-T> 



„ b 

tan/? =-7-, 

„ ab 
tanaXtan/? = -j2. 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS, igr 

Hence 

tan ^a+/?)=tan w 

a b 

m 

ab' 

a+b 

or r—Z f—ab=o. 

"^ ' tan w ' 

Resolving this adfected quadratic equation we find 

, a+b Ua+by 7 
' 2 tan w ^4 tan'^ w 

Having now found the focal length and the principal point, 
reference marks should be made on the edges of the metal box 
to indicate the horizon and principal lines as well as the focal 
length on the prints from the negatives. 

Measure the distance m, Fig. io6, Plate LII, from P to AC. 
From the corresponding comers A and C, Fig. io8, Plate LH, 
of the metal box lay out vt on AR and on CT. With a very 
fine and sharp file, held in the direction toward the lens, cut 
into the edge forming the rear frame of the metal box a clean 
and sharp notch at T and another ati?. 

Repeat the same operation from the comers A and B with the 
distance n from F to AB. 

The lines OQ and RT will be the horizon and principal lines 
of the photographs, when the camera has been leveled to bring 
the bubble into its proper position as mentioned in the foregoing. 

From R and T measure the distances Rr, Rr', Tt, Tf, equal 

to one haK of the focal length ( =~)- From O and Q measure 

Oo, Oo', Qq, Qq', equal to one quarter of the focal length, and 
at each one of these points make another notch with the file 



192 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

held in the direction of the lens. Every photograph will now 
show twelve triangular projections reaching into the dark border 
of the photograph. Four of these projections serve to fix the 
horizon and principal lines; the remaining eight give the focal- 
length value. 

2. Adjustment of Camera Spirit-levels. 

It now remains necessary to find the correct readings of the 
transverse levels (placed parallel with the sensitive plate) when 
the horizon and principal lines pass exactly through their cor- 
responding notches of the metal box. 

Set up the camera again, facing the same distant view as 
before, but in adjusting it bring the bubble of the transverse 
level near one end of the tube; note the reading of the level- 
tube graduation and expose a plate. When developed, it will 
give a horizon hne EF, Fig. 109, Plate LII, cutting the border 
of the negative in A and B, at some distance from the pictured 
notches O and Q. Now change the adjustment of the camera 
by bringing the bubble of the transverse level to the other end 
of the tube, note the reading of the level and expose another 
plate. This when developed will give another horizon line 
N'F', cutting the border of the negative in C and D. 

Great care should be exercised in both cases to keep the 
other level (the one at right angles to the sensitive plate) at its 
proper reading, in order to expose both plates while in vertical 
plan. 

After measuring CO and OA or BQ and QD, a simple pro- 
portion gives the proper reading of the transverse level which 
will bring the horizon line of the vertically exposed plate through 
the two notches O and Q of the metal box. 

The correct reading of the transverse level of the second 
set of levels is found by the same method, with the camera in 
the vertical position. 

All these operations must be executed with great care and 
precision (and with the help of a microscope of moderate power), 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 1 93 

as the subsequent iconometric plotting of pictured points is 
based upon the determination of the ordinates and abscissas of 
such points, on the pictures, with reference to the principal and 
horizon Unes which serve as a system of rectangular coordinates. 
It had been assumed that the levels were placed very nearly 
in correct adjustment by the maker as mentioned before. If 
found too much out, they must, of course, first be approximately 
adjusted by setting the metal box on a well-leveled plate. For 
this purpose the plate glass supplied with each camera may be 
set on the camera base and leveled like an artificial horizon. 

t 

3. Use of the Instruments Comprised in the Canadian 
Phototopographic Outfit. 

The instruments and tripod of the Canadian instrumental 
outfit being very light, steadiness may be secured by means of 
a net suspended between the tripod legs, into which a heavy 
weight (rock) is placed. With this device, photographs of good 
definition and better observations may be obtained than with- 
out it, and there is no risk of the instruments (secured to the. 
tripod) being blown over during one of the sudden and strong; 
gusts of wind so frequently encountered on elevated and exposed 
mountain peaks. 

After the phototopographer has arrived at a triangulation 
station he adjusts the transit and observes the azimuths and zenith 
distances of all signals (marking the triangulation points and' 
camera stations already occupied) in the vicinity that may be 
visible from his position. If accompanied by his assistant, 
each reads one vernier and records the readings independently 
of the other into separate record books. After the observa- 
tions at that station have been completed the two surveyors 
compare notes and any discrepancy that may be discovered in 
the recorded data is corrected on the spot by a careful repetition 
of the doubtful observations. 

The Canadian camera is carried in a sole-leather case which 
also contains twelve filled double plate-holders; when more 



194 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

holders are needed for a day's work they must bfe carried in a 
separate receptacle. Taking the camera from its case the level- 
ing base, Fig. 103, Plate LI, is secured to it by means of the 
central clamp-screw, and the camera is then placed upon the 
tripod (from which the transit had been removed) without dis- 
turbing the position of the latter. 

The shade or hood H, Fig. 99, Plate XLIX, is now unfolded 
and attached to the hooks at the front of the camera. A plate- 
holder is inserted into the carrier and the number of the plate, 
in position to be exposed, is noted upon a rough outline sketch 
of the panorama views (commanded by the field of the camera 
image as indicated by the converging lines cut into the exterior 
sides of the camera-box), entering also such notes as may be 
of value for the subsequent development of the plate, for the 
iconometric plotting of the topography photographed upon the 
latter, or for the lettering of the finished map. 

Having made sure that the cap is secure on the lens, the 
slide is withdrawn from the pla,te-holder and the sensitized sur- 
face of the plate is brought into direct contact with the frame of 
the metal box by turning the screw G, Figs. loi and 102, Plate LI, 
devised for this purpose. The surveyor next turns the camera 
in azimuth until the lines on the upper face of the wooden casing 
show that it is properly directed, or oriented, to include the 
panorama section to be photographed between the lines. The 
field of view should, of course, in each case coincide with the 
outUne sketch bearing the number of the plate in position to 
be exposed. Sighting along the lines (up and down) shown 
on the side face of the wooden camera casing, the observer can 
assure himself whether the view on the image plate reaches high 
or low enough to control the landscape ; if it does not, he either 
puts the longer dimension of the camera upright, or, if already 
in that position, he may have to occupy a secondary camera 
station, either above or below the one occupied, as the case 
may be. 

The observer next levels the camera carefully in the manner 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 195 

previously described and exposes the plate. Whenever the 
sunlight appears inside of the front hood, the latter should be 
shaded off during the exposure of the plate by holding some 
object (plate-holder slide or hat) above the hood. Under no 
circumstances must the sun be allowed to shine upon the lens. 

Every evening after the return to camp the surveyor replaces 
the exposed plates (in the dark-tent) by new ones, using a ruby- 
colored light for this purpose. He also marks the exposed plates 
in one corner close to the margin, before removal from the holder, 
with his initials, with the number of the dozen and number of 
plate, using a soft-lead pencil for this purpose. N. N. — IV — 5, 
for instance, means plate No. 5 of the fourth dozen and exposed 
by N. N. This would be the forty-first plate exposed in that 
season. 

The exposed plates are placed into a double tin or copper 
box. Fig. no, Plate LII, which can be closed hermetically and 
which will float when filled with two dozen plates (should it 
be accidentally thrown into water by the capsizing of a canoe). 
These boxes, as soon as filled, are shipped to the head office 
in Ottawa, where the plates are developed by a specialist. 

The data obtained by aid of the transit theodolite for tri- 
angulation purposes are recorded in the field-books in the man- 
ner customary for such work. 

The horizontal angles observed with the transit (or altazi- 
muth) to the terrene points (so-called "reference points ") 
marked on the outline sketch, which should accompany each 
negative, serve not only for the orientation of the horizontal 
projection of the plate on the working-plan — ^to " orient " the 
so-called " picture trace " — but they also aid materially to ascer- 
tain the amount of (and to counteract in a measure) the dis- 
tortion of the paper prints or photographic enlargements. The 
vertical angles, together with the plotted distances i to such refer- 
ence points, serve to check and verify the position' Of the horizon 
line on the different photographs as given by the camera alone. 

Important camera stations are occupied by the surveyor, 



196 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

secondary stations by the assistant with a separate camera. No 
trigonometrical observations are made by the assistant when 
occupying secondary and tertiary camera stations, as he is not 
supplied with a transit. The surveyor locates such stations by 
observing upon the signals, erected by the assistant before leav- 
ing the station, from his own stations, and he subsequently com- 
putes their positions as " concluded points." All views are 
taken with the same stop, //36. 

C. The U. S. Coast and Geodetic Survey Phototopographic Cameras. 

In the preceding we have already referred to the desirability 
that phototopographic surveying instruments designed for use 
in rough mountain districts, where transportation facilities are 
restricted and generally confined to portage over rough trails 
(or to transportation up steep mountain sides on the backs of 
packers), should be made as simple as possible to indefinitely 
remain in perfect adjustment. 

The phototopographic party generally reaches the mountain 
station, after an exerting^ chmb of several hours, in a more or 
less fatigued condition, and to obtain the best results the observer 
should not be required to spend much time in assembUng and 
adjusting the instrument before the actual survey work may be 
begun. Then, too, the atmospheric conditions are rarely stable 
for any length of time, making it most desirable to utiUze favor- 
able conditions at once. The fields of labor in S.E. Alaska, where 
the Coa.st and Geodetic Survey camera work has been done, 
are peculiarly well adapted for the application of the phototopo- 
graphic methods, on account of the prevailing cloudy condition 
of the atmosphere in the higher altitudes. Distant peaks and 
mountain groups, without apparent warning, become suddenly 
shrouded in drifting mists, which soon gather into a thick cloud 
stratum, shutting the mountains out from view for days and 
weeks at a time. During the summer months the prevailing 
southerly, vapor-laden winds drifting inland from the Pacific 
Ocean find their moisture condensed on approaching the snow- 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. I97 

and ice-fields or hanging glaciers in the higher altitudes. Clear 
days are generally accompanied by calms or they occur when a 
northerly drift in the air-currents prevails. 

Phototheodolites or instruments in which the elements of a 
transit and a camera are assembled into a single apparatus, all 
parts being merged into a composite instrument, to remain 
united during the various operations of observing and plate 
exposures, mostly have the objectionable feature of unstable 
adjustments, requiring frequent tests and readjustments of their 
component parts. They are, furthermore, more or less cumber- 
some and heavy, making them more liable in transportation 
than instruments that may be divided into two or three parts 
(each section being complete in itself) and carried by two or 
three helpers. 

The original type of the U. S. Coast and Geodetic Survey 
camera, used in connection with the topographic recormaissance 
made in S.E. Alaska under the U. S. Alaskan Boundary Com- 
missioner, was similar in form to Capt. Deville's original survey- 
ing camera, except that it was provided with a separate- tripod 
with ball-and-socket adjustment and that the teeth or index 
marks which serve to fix the principal and horizon lines on the 
negatives could be pressed into direct contact with the sen- 
sitized film of the photographic plate simply by turning a button 
after withdrawal of the slide. This camera was provided with 
a ground glass, enabling the observer to inspect the entire field 
controlled by each plate before exposure was made. 

The camera proper was a plain rectangular box, made of 
well-seasoned mahogany, 6fx6|X9J inches in size. It was 
always used in the same position, with the short faces vertical. 
A circular level attached to the upper camera side served for 
leveling the instrument, bringing the photographic plate into 
vertical plane. 

The bamboo tripod legs were made in three sections, each 
sixteen inches long and screwed together at the joints. When 
dismembered, the tripod was carried in a sole-leather case or 



198 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

knapsack, together with the camera, six double plate-holders, 
note-book, barometer, etc. A yellow color screen could be 
attached to the inner side of the camera-box just behind the 
lens mount. The four-inch transit used in conjunction with 
this camera had a separate tripod. 

With a view toward simphcity in structure and a light weight 
to be transported in the mountains, the new Coast and Geodetic 
Survey phototopographic instrument has been made into three 
distinct parts or sections, the transit, the camera, and one tripod, 
serving for both. 

The superstructure, embracing the Y's, telescope, and ver- 
tical circle, may be lifted off the horizontal circle, to which it 
ordinarily is secured by two capstan-head screws, uniting the 
base-plate of the Y support with the vernier plate of the hori- 
zontal circle. Plate XCVIII shows the transit as used for 
trigonometric observations. 

The camera, complete as such, may be mounted on the ver- 
nier plate of the horizontal circle with the same capstan-head 
screws' that secure the superstructure of the transit. Plate XCIX 
shows the camera-theodolite in its usual position (long sides of 
the photographic plate horizontal). The truncated aluminum 
cone under the camera-box is secured to the latter by means 
of a central clamp-screw (within the hollow cone), and the 
base-rim of the cone is then fastened to the vernier plate of the 
horizontal circle with the two capstan-head screws already men- 
tioned. 

Both transit and surveying camera are used on the same 
tripod. The understructure (with the horizontal circle) is con- 
nected with the tripod by means of the triangular tripod plate 
shown in upper part of Plate XCIX. This triangular plate is 
screwed to the tripod and the three leveling-screws of the under- 
structure are placed on the arms of this plate, a clamp device 
securing the conical ends of the leveling-screws to the tripod 
plate serves to prevent a possible disturbance of the under- 
structure when the exchange fronl transit to camera is made. 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. I99 

The adjustments of transit and camera are stable and with 
ordinary care will suffer no frequent changes. To reduce weight 
aluminum has been used when practicable without sacrificing 
rigidity and strength. 

The camera is packed in a stout packing-case, together with 
eight double plate-holders, focusing-cloth, note-book, etc. The 
transit is packed by itself. 

A small triangular net or hammock, that may readily be 
attached to the legs, should be provided when stations are occu- 
pied in windy weather. These light instruments lack stability, 
but by placing a suitable weight (a rock will do) in the net sus- 
pended between the tripod legs no noticeable vibration will occur. 

The inner edge of the rear frame of the inner (aluminum) 
camera-box is supplied with notches to mark the horizon and 
the principal lines. The constant focal length (about S^V32 
inches) of the lens is also laid off on the inner edge of this rim, 
one half to either side of the principal line and one half to one 
side of the horizon line. All these notches will be printed on 
the edges of the negatives, giving ready means for checking 
distortions in the prints. 

As will be noted, this camera-box is similar in form to Capt. 
Deville's new model, having also the same lens (Zeiss anastigmat, 
6^X8^, series V). The plates used in this camera are 5X8 
inches. The outer box or casing is made of well-seasoned J-inch 
mahogany reinforced with strips of brass. The sensitized film 
of the photographic plate may be brought into direct contact 
with the rear frame of the inner camera-box by means of a milled- 
head screw attached to the front board of the outer box with a 
counteracting spiral spring similar to G, Figs. loi and 102, 
Plate LI. 

The lens is about 2^/16 inches from one long side and 3^Vi6 
inches from the other long side of the outer wooden case, making 
the horizon line correspondingly nearer one long side of the 
camera. When the main field of the terrene falls below the 
station the camera is mounted with the lens low and vice versa 



200 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Three sets of cross-levels are attached to the inner camera-box, 
each being covered with a glass window inserted into the outer 
wooden case, so that a set of cross-levels will appear on the upper 
camera side for each of the three positions in which the camera 
may be mounted. Plate C shows the vertical position of this 
camera; it is used when the terrene to be pictured subtends, 
rather large vertical angles for both elevation and depression. 

D. L. P. Paganini's neic Phototopographic Instrument for Re- 
connaissance Surveys on Scales 0} 1 150000 and 1 :i 00000 
(Model of 1897). 

To overcome the difficulties encountered in the topographic 
reconnaissance work (i : looooo scale) in Eritrea (East Africa) — 
due principally to the torrid climate — and in Sardinia (i : 50000 
scale) — on accoimt of the danger of contracting malarial fevers — 
L. P. Paganini has devised another surveying-camera, smaller, 
more simple in form, and easier in manipulation than the type 
just described. 

This instrument (model of 1897) combines rapidity in the 
field operations with a minimum expenditure of money, and it 
materially reduces the period during which the operator has 
to be exposed to the vicissitudes of climate and weather. It is 
compactly built, essentially light in weight, and preserves the 
various adjustments, when once made, almost indefinitely, at 
least for a long time if the instrument is carefully handled. 

This instrument, together with all accessories, compass, 
frame, tripod head, shutter, dark-cloth, etc., may conveniently 
be packed into a knapsack to be carried by a single packer, the 
entire outfit weighing only about 15 kilogranmies. 

This photogrammeter is composed of the following parts: 

First. A photographic camera; 

Second. A horizontal graduated circle attached to the ver- 
tical axis of rotation, with superimposed alidade bearing 
verniers and spirit-levels ; 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 201 

Third. An azimuth compass; 

Fourth. A tripod with folding legs which may easily be 
taken apart. 

I. The Phototopographic Camera Proper. 

The camera of Paganini's latest photogrammeter is rigidly 
•constructed of aluminum. It has been given a prismatic form 
with an equal-sided trapeze for base. The rear side of the 
camera-box is formed by a metal frame which supports either 
the ground glass or the sensitized photographic dry-plate. The 
plates are 18X24 cm. in size and they are exposed with the longer 
side horizontally. 

Attached to the center of the camera front is a metal collar 
or tube into which a tube may be screwed having a thread of 
I mm. rise and holding the objective in the outer end. This 
objective tube is provided with a flat ring soldered near the 
objective end to the tube in such manner to leave a cylindrical 
space between the ring and objective tube into which the fixed 
collar — attached to the front side of the camera — may enter 
when the objective tube is screwed into the camera collar. The 
latter has a millimeter graduatioii on its outer surface extending 
in the direction of the optical axis. The rear (beveled) edge 
of the flat ring is divided into ten equal parts, and when the 
objective is brought nearer to or farther from the image plane — 
by revolving the lens tube about its axis — the beveled edge of 
the flat ring wiU slowly be moved over the miUimeter scale. Fur- 
thermore (this beveled edge being divided into ten equal parts), 
the position of this circular scale on the beveled edge with refer- 
ence to the longitudinal millimeter scale will permit the focal 
length to be read to tenths of a millimeter for any position of 
the lens. 

The objective of this camera is a Zeiss wide-angle anastigmat 
with a principal focal length of 182 mm. With a small dia- 
phragm stop it produces a picture of 40 cm. diameter, equivalent 
to an angular field of 104°. With the diaphragm aperture //35 



202 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

it covers a plate of 20X26 cm. very well, and as the adopted 
size of plate is only 18X24 cm. we obtain a very clear and sharp 
definition over the entire plate even when using a larger dia- 
phragm opening. 

The perspectives obtained with this camera command an 
angular field of 67° horizontally and 53° vertically. With six 
plates we can, therefore, cover an entire panorama view (of 360°) 
from which vertical angles of elevation or depression may be 

deduced up to ^^ ( = 26° 30'). 

Two adjoining plates have a common vertical margin of 
3° 30' in width. Horizontal angles between points falling within 
this panoramic zone or belt of 53° angular width may also be 
deduced from the six plates iconometrically. 

This instrument has been carefully constructed with optical 
axis of the camera perfectly normal to the image plane, and 
the point of intersection of optical axis with the image plane — 
the principal point — is photographically transferred to every 
photographic perspective as the point of intersection of the pic- 
tures of two very fine silver wires crossing each other at right 
angles. These wires are secured, quite close to the image plane, 
in such manner to be easily removed and replaced by others 
in case they should accidentally be ruptured.^Directly below 
the camera-box, attached to its lower side or base, are three 
Z-shaped metal arms with rectangular bends. One is placed' 
immediately below the objective and the other two in the rear, 
below the cross-wires. The lower (horizontal) arms of these 
metal Z bars are equidistant from the camera base, and they 
are perforated by smooth circular openings which readily receive 
three stout screw-bolts securely and permanently attached at 
right angles to the alidade of the horizontal circle. Each one 
of these bolts has two nuts of which the lower one supports its 
corresponding Z-bar arm, while the upper nut has been added 
as a locking device to secure the position of the lower nut together 
with the corresponding Z-bar arm at any desired height, after 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 203 

the proper adjustment of the camera's position on the horizontal 
circle has been made. 



2. The Horizontal Graduated CrccLE. 

The horizontal circle of this photogrammeter has a diameter 
of 14 cm.; it is graduated into half-degrees and reads from o° 
to 360°. The vernier reads to single minutes, but 30 seconds 
may be estimated. The vertical axis of rotation of the instru- 
ment projects above the plane of the alidade and the latter is 
secured in a plane at right angles to the axis of rotation by means 
of a stout collar covering the latter. 

The instrument is supplied with three leveling-screws, clamp- 
and tangent-screws, spirit-levels, etc., the general arrangement 
being the same as for any other surve)dng-theodolite. 

The metal tripod head supports a circular spirit-level for 
an approximate adjustment into position of the vertical axis, 
the final leveling being accomplished by means of the three 
leveUng-screws, which form the support of the instrument on 
the tripod head, together with a pair of cross-levels attached 
to the alidade surface. 

The superstructure is secured to the tripod head, precluding 
an accidental upsetting, by means of a central clamp-screw with 
spiral spring and a long handle for an easy manipulation between 
the tripod legs, the same as shown for the other ItaUan photo- 
grammeters. 

3. The Azimuth Compass. 

On the upper surface of the camera-box is a cylindrical recep- 
tacle of pure aluminum, to receive the magnetic compass which 
is modeled after the so-called Dixey or Schmalkalder pattern, 
so well adapted for topographic work. It has the usual pris- 
matic eyepiece and vertical hair-sight, which are permanently 
fixed in the vertical plane which passes through the optical axis 
of the camera objective. 



204 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

4. The Tkipod. 

The tripod of this photogrammeter is practically the same 
as has been described for the other Italian phototheodolites, 
except that the legs are folding and may be carried together 
with the instrument by one packer. 

5. Adjustments and Use of this Instrument. 

Before commencing the regular phototopographic survey of a 
given area with this instrument, certain observations should 
be made which, however, will serve equally well for all sub- 
sequently occupied panorama stations, and certain adjustments 
should be made or verified which, owing to the stable and solid 
construction, will also remain in stabiliment for a long time 
thereafter. 

Of course, the operator should, after all the adjustments 
have once been made, check them from time to time, particu- 
larly when it may be supposed, or when it is known, that the 
apparatus had been subjected to unavoidable shocks or care- 
less handling by packers while in transit over rough roads or 
trails. With an adjusted instrument and careful handling the 
final adjustment that will be necessary to be made before occupy- 
ing the camera station for observing reduces itself to a small 
matter. 

The first step when the phototopographic survey of a given 
area is to be made consists in a most accurate determination 
of the constant length of the focal distance, which is preferably 
done on a bright day in the following manner: The objective 
is moved back or forth until some far-distant but well-defined 
points of the terrene appear well outlined and quite sharp in 
definition on the ground-glass plate under the dark-cloth, using 
a small magnifying-glass to ascertain the position of the plate 
(the focal length) giving the best definition. It is advisable to 
focus in this manner successively upon several well-defined 
distant points, reading the scale indications for each point focused 



SURVEYING-CAMERAS WITH CONSTANT FOCAL LENGTHS. 205 

upon and entering the records in the field-book. We will thus 
obtain a definite value for the principal focal length, as indi- 
cated by the mean of the recorded scale-readings. 

We next determine the diaphragm aperture which is to be 
used for the entire panorama, selecting the smallest stop that 
will yet give a uniformly good definition for the entire field con- 
trolled by the plate — 60° in azimuth and 45° in altitude — having 
special reference to a good definition of the summits of distant 
mountains that may be showti on the perspective. 

The adopted diaphragm aperture — the instrument in ques- 
tion has a revolvable diaphragm disc with numbered apertures 
of graduated sizes — at different hours of the day, at different 
altitudes, and under different illumination (" light-intensity ") 
will only, in part control the length of exposure, as the latter also 
depends in great measure on the, rapidity (" sensitometer num- 
ber") of the plate that may have been selected for use in the 
survey, and on the so-called rapidity of the lens, together with 
the color screen, if such is used. 

A systematic exposure of a few trial plates under different 
conditions regarding hour of the day, eleva:tion of station above 
sea horizon, and illumination^ens, diaphragm aperture, and 
plate remaining the- same — will give valuable data for future 
reference and for judging the required length of exposure cor- 
rectly under similar conditions. 

One indispensable condition to be fulfilled when proceeding 
to make the final adjustments of this instrument would "be to 
make a complete turn of the camera about the verticaL axis, 
very much the -same as when exposing the plates for U cbnip'lete 
panorama. If the instrument is well assembled with all its 
parts in rigid adjustment, and if the tripod is securely placed in 
position on firm ground and no disturbing causes interfere, the 
instrument should, while being thus rotated, maintain its axis 
(and its image plane) perfectly vertical. ' 

To realize ' this condition the cross-levels ' attached to the 
upper surface of the ahdade should be well adjusted. This 



206 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

may be accomplished by means of the three leveling-screws 
which transverse the tripod head and form the support of the 
superstructure, in precisely the same manner as such adjust- 
ment would be made with an ordinary surveying-theodolite. 

After establishment of the verticaUty of the axis of rotation 
there still remains the adjustment into position of the rigid system 
of the three orthogonal axes, which have their common origin — 
point of intersection — in the principal point of the perspective. 
This system of coordinates is composed of: 

First. The optical axis of the photographic camera, repre- 
senting the principal ray of the photographic perspective. 

Second. The two axes intersecting each other at right angles — 
one horizontal and the other vertical — which are visible 
on the groimd glass and which are photographically trans- 
ferred to the photographic perspective, where they repre- 
sent the horizon and the principal line respectively. 

The rectangular intersection of these two Unes (last named) 
is perfectly obtained by the instrument-maker. Two very fine 
silver wires are strung quite close to the image plane, where 
they are held in position by thin metal plates, the required ten- 
sion being applied by means of pressure-screws. The correct 
position of these cross-wires with regard to the rear metal frame 
of the camera-box is assured by four fine incisions made with a 
dividing-machine into the rear metal surface of the frame against 
which the grotind-glass plate or the sensitized photographic 
plate rests when they are in position. 

The degree of accuracy with which this rigid system of axes 
is placed into position (and the degree of accuracy in their direc- 
tions regarding horizontality and verticality) in a great measure 
controls or determines the attainable accuracy in the iconometric 
plotting, based upon pictured points referred to that system 
of lines as coordinates. 

The optical axis of the camera, which by construction inter- 
sects the perspective plane — the image plane — at right angles 
in the principal point, should be horizontal, and therefore the 



SURVEYING-CAMERAS WITH CONSTANT TOCAL LENGTHS. 207 

optical axis should intersect the plane containing the two axes 
of coordinates- (the cross- wires) also at right angles. It is fur- 
thermore required that the wire representing the horizon line 
of the perspective be horizontal, or, in other words, the optical 
axis of the camera and the horizon Une of the perspective are 
to be in the same horizontal plane — that is, in the horizon plane 
of the camera station. 

This condition may easily be fulfilled by using the screws 
which support the three Z bars as three leveling-screws. The 
three lower arms of these Z bars are first placed approximately 
at equal altitudes above the plane of the alidade circle (which 
previously had been leveled) by means of the supporting nuts, 
having first loosened the upper counter nuts to permit the Z-bar 
arms to move freely over the upright screw-bolts. Now observe 
the image on the ground-glass plate and bisect a distant point 
with the pictured intersection of the cross-wires. Change the 
pointings to the distant bisected point to different positions along 
the line defining the horizon by revolving the camera in azimuth 
from left to right, or vice versa, at the same time raising or lower- 
ing the screws which support the rear Z-bar arms of the camera,., 
until a point is found bisected by the intersection of the cross- 
wires, which, when turning the camera about the axis of rotation, 
does not leave the horizon wire, rather continues to be bisected 
by that wire while being moved over it from one extremity to 
the other, falling neither above nor below that wire, during the 
full azimuthal swing through the entire length of the ground 
glass. ' 

If this distant bisected point was not in the horizon plane' 
of the camera-theodoUte, the curve which it describes on the 
ground-glass surface — during the revolution of the camera through; 
an azimuthal field of 60° — will be traceable with the eye, the 
point while in transit will pass over the vertical wire above 
or below the point of intersection of the two cross- wires, according 
to its position in nature, whether below or above the horizon 
plane of the instrument. 



2o8 PHOTOTOPOGRAPHIC METHODS AND INSTilUMENTS. 

The farther removed from the intersection of the cross-wires 
the crossing of the pictured distant point over, the vertical wire 
is the greater will be the. curvature of its hyperbolic trace on 
the ground-glass plate and. the greater will be the vertical dis- 
tance , between the bisected point and the horizon plane. 

From the preceding remarks it will be evident that the Z bars 
supporting the camera in the rear may be adjusted by simply 
watching the courses of points of different elevation as they are 
traced (by the points while in transit) on the ground glass during 
the azimuthal swing of the camera. If the distant point, having 
the same elevation as the optical axis of the camera, leaves the 
horizon wire and passes over, the vertical wire above the inter- 
section of both, the supporting . Z bars (those under the rear 
frame) are to be lowered and vice versa. The distance between 
the point of crossing and the point of intersection of the cross- 
wires gives .a measure for the amount of change to be made in 
the elevation of the rear Z-bar supports. 

After a few carefully made trials, the position of the horizon 
wire will be such that any point bisected at one extremity of 
the horizon wire will continue to be bisected by that wire during 
the revolution of the camera through a horizontal field of 60°. 
When this has been accomphshed the counter-screws of the 
upright screw-bolts are tightened to secure (the Z bars) . the 
camera in this position with reference to the horizontal circle. 

This adjustment once made and secured will be maintained 
undisturbed for a long time and the horizon wire will now coin- 
cide with the horizon line of the photographic perspective for 
all positions of the optical axis, when the camera is rotated about 
its axis during the exposures of the plates comprising a pano- 
rama, provided, of course, the phototheodolite's axis of rotation 
has remamed vertical. 

The final adjustments to be made in the field after the 
phototheodolite has been placed in -position at a carefully 
selected phototopo graphic station may now be summed up as 
follows : 



SURVEYING-CAMERAS AKD GEODETIC INSTRUMENTS. 209 

First. Adjust the camera's objective to make the principal 
focal length, previously ascertained and recorded in the 
field-book, agree with the reading on the' "objective 
scale." 
Second. Verify the position of the rotary diaphragm in order to 
have the particular alpertiire in position that may be required 
for the particular braiid of plate under the particular 
conditions of the atmosphere at the place occupied. Also 
verify the setting of the time-shutter to see that the expos- 
ure vlfill be correct for this particular diaphragm aperture, 
plate, illumination, altitude, and subject, although the 
latter plays a minor r6le here, as it seldom varies much in 
phototopography. 
Third. Verify the level adjustments and see that the axis 
of revolution of the instrument is vertical, as the hori- 
zontality of the optical axis, ' the verticality of the image 
plane and of the " principal" wire, as well as the hori- 
zontality of the " horizon " wire, depend thereon. 
The preceding descriptions shdw that much has been done 
in Italy towards pushing photographic Surveying to a high state 
of perfection, and we are particularly indebted to Paganini for 
nunierous improvements in phototbpographic and iconometric 
instruhients, including methods of their use for topographic and 
hydrographic surveys. Pagariini's gobd results, his experience, 
and advice have materially aided in the decision in favor of the 
phototopographic method in a numbei: 'of surveys, particularly 
for the strvey of the mountains of the Caucasus (and Tiflis) 
under the direction of Baroii von Steinem. " ' 

III. Surveying-cameras Combined with Geodetic Instruments. 
(Phototheodolites, Phototachymeters, Photographic Plane 
Tables,, etc.) 

The data obtained in the field with the photographic-sur- 
veying cameras considered in the preceding paragraphs had to 



210 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

be supplemented with instrumental field observations to gain a 
complete topographic control of the territory traversed by the 
phototopographic surveying party. 

The idea of combining surveying instruments with a camera 
into a compact and serviceable apparatus originated very early 
with phototopographic workers. Refined phototheodolites are 
to this day the favorite photographic-surveying instruments not 
only in Europe, but they are also widely in use in other countries, 
particularly when these are dependent on European mechanicians 
to supply the demand for instruments of precision. 

Phototheodolites have been devised to secure great precision 
in the results obtained with them; refined methods are largely 
employed in the field observations in the culling of data from 
the photographic perspectives, and in the computations. 

Generally speaking, the best results for topographic pur- 
poses are obtained by methods that have been devised with 
due reference to the fact that phototopography essentially 
and primarily is a constructive or graphic art, based on graphic 
or pictorial records, in the form of perspectives, that are to 
be transposed into orthogonal projections in horizontal plan, 
instrumental observations being required only to furnish such 
elements as may be needed to facilitate the graphic trans- 
position of lines of direction and distances, to insure accu- 
racy, and to obtain certain checks or a proper control for the 
work in its entirety. 

It has already been pointed out in the preceding chapters 
that phototopography is based essentially on the same methods 
which are followed in topographic plane-table surveys, and the 
best results may be expected when the surveying-camera is used 
with the same object in view which the plane-tabler essays to 
obtain. To increase the degree of precision the plane-tabler 
will occupy a greater number of stations, and similarly in photo- 
topography any degree of accuracy may be attained by increasing 
the number of camera stations for any given area. 

Photographic surveys have been conducted principally in 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 211 

regions where other surveying methods are either precluded or 
where their application would entail great cost and consume 
too much time and such regions are characterized chiefly by 
a rugged and broken topography. The necessity, therefore, 
lies close at hand to devise instruments which will not easily get 
out of order or drop to pieces when transported over rugged 
mountain trails; the more simple their structural composition 
the better adapted they will be for the production of rapid and 
accurate work. 

In Europe phototopography has generally been employed 
for surveys plotted on large scales, necessitating the occupation 
of numerous stations, with a resulting slow progress from one 
locality to another. Then too the instrumental outfit could 
readily be brought very near, if not actually to the very place, 
where the work was to be done, by convenient and safe means of 
transportation. The instruments are very seldom exposed to such 
primitive and rough methods of transportation over long dis- 
tances, as generally has been the case on our continent when 
surveying- cameras have been used. 

It is evident that the combination of a camera and a surveying- 
instrument into a well-united, well-balanced, easily manipulated, 
and essentially light and withal rigid instrument is not easily 
accomplished. It is not surprising, therefore, that we meet 
with a great number of types of phototheodolites and other photo- 
grammeters in which the difficulties in construction have been 
overcome, more or less successfully, by various devices. In 
the following we shall describe the principal types of photo- 
graphing-surveying instruments that are either of historical 
interest or are in use at this date. 

A. L. P. Paganitii's Photo grammetric Instrument {Model of 1884). 

The Italian photogrammetric apparatus devised by L. P. 
Paganini, model of 1884, is illustrated in Figs, iii and 112, 
Plates LIII and LIV. It is supported by a tripod which may 



212 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

be dismembered into the tripod head H and three " alpenstocks " 
A. The instrument propfer may be separated into two parts, 
the camera-box C and the Y supports with eCcentricailly located 
telescope T. 

S', S', S' indicate the 3-foot screws — only two are visible in 
the illustrations on Plates LIII and LIV — ^which form 
parts of the tripod head H and which serve to level 
the theodolite. 
Si, S2, S3 represent three leveling-screws which support the 
camera proper and which serve to adjust the position 
of the cross- wires affixed to the rear frame of the 
camera-box. The camera C is connected With the 
upper limb of the theodolite by means of a catch- 
lever K in such manner that the azimuthal revolu- 
tion of this limb will also rotate the camera hori- 
zontally. 

L is a spirit-level attached to the telescope T, both being 
supported by an upright or Y support U secured at 
right angles to the horizontal limb of the' theodolite 
and at one side of but close to the camera. 

T is an ordinary surveying- telescope (astronomical) 
and it is provided with the usual cross-hairs (one 
vertical and the other horizontal), adjustable in 'the 
■customary manner. 

C represents the camera-box. It is made of hardened 
pasteboard, which is strengthened by a nietal skele- 
ton frame or casing B. ' 

The camera is supplied with an aplanatic objective 
(" antiplanat "), made by Steinheil, having a focal 
length of 244.5 i'^™- 
The aperture in the diaphragm has a diameter of 5 mm. 

Regarding the general arrangement of this instrument it may 
be said that: 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 213 

First. The optical axis of the photographic lens (objective) 
is parallel with that of the telescope T and it always is 
perpendicular to the picture or image plane. 
Second. The intersection of the optical axis of the camera 
and picture plane — the principal point of the photographic 
perspective — is marked by the point of intersection P, 
Fig. 113, Plate LV, of two very fine and adjustable plati- 
num wires 00' and //' securely fastened to the rear frame 
of the camera-box, very close to the image plane. 
When the instrument is leveled and in adjustment one 
of these fine wires {00') will be horizontal, while the other 
(//') will be vertical, and both will be in a (vertical) plane 
parallel to the image plane. 
The optical axis of this camera may be brought into hori- 
zontal plane by rotating the same about the horizontal axis CC, 
Fig. Ill, Plate Llll, and clamping the screw h. In this position 
the image plane and plane containing the platinum cross-wires 
00' and //' will both be vertical. 

The horizontal wire 00' may be adjusted into horizontal 
plane, after the instrument has been carefully leveled, by find- 
ing ■ some easily identified and readily recognized point on the 
ground-glass plate, which is bisected by this wire 00', and by 
gently revolving the camera in azimuth. If the wire 00' is in 
horizontal' plane, the observed point will be seen to move over 
the entire length of the wire while the revolving motion is given 
the camera. Should the bisected point, however, appear above 
or below the wire 00' at any time during the azimuthal revolu- 
tion of the carhera, the same will have to be adjusted into hori- 
zontal plane by aid of the two front screws ^2 and ^3, Figs, in 
and 1 1 2 , Plates LIII and LI V. 

The camera is provided with a short tangent or slow-motion 
screw t, by means of which the same may be slightly moved 
in azimuth, while the telescope T and horizontal limb of the 
theodolite remain stationary. This ■ arrangement will enable 
the observer to place tlie optical axis of the camera parallel to 



314 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

that of the telescope T, provided both had been adjusted in hori- 
zontal plane. This correction is made by " pointing " the tele- 
scope to some well-defined distant point and clamping the the- 
odolite in this position. The camera is now moved by means 
of the tangent-screw t to the right or left until the same point 
appears in the intersection P of the two camera wires 00' 
and //'. 

The prints of the camera cross-wires 00' and jf appear on 
every negative taken with this instrument, and as their plates 
were exposed while vertical to the optical axis of the camera, 
the perspectives that are obtained (after the instrument had 
been adjusted as described) are in vertical plan, each showing 
the principal point of view P, as well as the principal and horizon 
line /'/ and 00', intersecting each other in P at right angles. The 
horizon line 00' on the picture represents the intersection of the 
horizon and picture plane. All points on the picture bisected 
by the horizon line have the same elevation (disregarding the 
error due to curvature and refraction) as the optical axis of the 
camera at the station whence the picture was taken. 

In place of the fixed platinum wires some photographic- 
surveying instruments have four sets of teeth (or a series of notches) 
attached to the rear frame of the camera-box, close to the picture 
plane (Fig. 114, Plate LV). If prints are used for the map 
construction instead of the plates, this arrangement is preferable 
to the fixed wires, as the latter often obscure details and as the 
prints may be distorted to such a degree that the lines 00' and 
//' may have to be corrected, thus giving two sets of lines across 
the face of the print. When only the ends of the cross-wires are 
indicated on the pictures by means of the teeth, the correct posi- 
tions of the cross-lines may be ascertained or checked experi- 
mentally and the Hnes are then drawn across the face of the 
picture by very fine lines in red ink. 

Great care should be exercised in the proper location of those 
lines, as they fomi a rectangular system of coordinates to which 
every pictured point that is to be mapped must be referred. They 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 215 

also play an important part in ascertaining the value of the focal 
length of the picture, which is one of the principal elements 
required in iconometric plotting. 

Fig. 112, Plate LIV, shows the camera in a position to take 
a picture of terrene so far below the camera horizon that the 
plate when exposed in vertical plane would not "take it in." 
The construction of the instrument will permit a depression 
(or an elevation) of the- optical axis of 30° below (or above) the 
horizon by loosening the clamp-screw and revolving the camera 
about the secondary axis of rotation. (See paragraph on in- 
clined picture plates.) 

Constant Focal Length of the Italian Cameras. — Fig. 115, 
Plate LV, shows the longitudinal section of a surveying-camera 
with the diaphragm AB in position between the lens doublets. 
The aperture of the diaphragm is taken as 5 mm. in diameter. 
Only such rays of light emanating from a point N in nature 
will reach the point n on the image plate II that form a cone 
about the central ray nON as axis, with apex in n and base in O. 
For the case illustrated in the diagram (Fig. 115, Plate LV), 



that base will be an ellipse with 5 mm. length for the short axis, 
while a pencil of light emanating from a point C on or very close 
to the optical axis of the objective would be intercepted by the 
plane of the diaphragm AB in a, circle of 5 mm. diameter. 

The Italian lens is so focused that even for the largest aperture 
of diaphragm used, all points from 10 meters to infinite distance 
from the camera-lens O, Fig. 115, Plate LV, will be clearly 
photographed with a maximum error in definition of 0.06 mm. 
(for 10 m. distant objects). ( 1 1^ 

If a = distance of object from the point O (10 meters to infinite 
distance); /; '' 

/ = principal focal length of the camera (240 mm.); 
6= focal distance, variable for different lengths of a; 
we find from the well-known relation 

III 
J^a^J' 



2l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

a-} 

^r 

By adopting _a^«^ mm. as value for /, and substituting different 
values, from i meter to 300 meters, for a in the preceding formula, 
we obtain the following values for 6 : 

a (in m.) = i lo 20 30 40 50 75 100 200 300 co 
b (in mm.) = 3i5.8 245.9 242.9 241.9 241.4 241. i 240.7 240.51240.2 240.02 240.00 

The error, therefore, in maintaining the focal distance constant is 
6 mm. if the object is lo meters distant from the nodal point; 
it is I mm. if the object is 50 to 100 meters distant and it is in- 
appreciable if the object is 3oo,m. or more distant from the nodal 
point of the camera-lens. 

/x\ ■ f 

The value I — I of the error (lack of definition or distortion) 

produced in the photograph; for points or objects at different 
distances (a), when maintaining a constant focal length, may be 
seen from the following : Assuming again that the image plane //, 
Fig. 115, Plate LV, be held in a fixed position and 240 mm. 
distant from the nodal point of the lens, it will be evident that the 
image plane // (Fig. 115, Plate LV) will intersect some of the 
light pencils or cones of rays (passing through the aperture of 
the diaphragm) in a circle (or in an ellipse) instead of intercepting 
their apex. We see from an inspection of the foregoing table that 
this circle of diffused light will increase in size with a decreasing 
distance {a) of the object to be photographed. The true point 
would be the center of the circle and the length of its diameter 
a; may be . ascertained from the following relation (Fig. 116, 
Plate LV): 

x:0=f:a, 

fO 

«=' — . 
a 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 21 7 

Again assuming (Fig. ii6, Plate LV) 

1=240 mm. (principal focal length), 

a=i meter to infinite distance, and 

O = diameter of aperture in diaphragm = 5 min/^ 

we find the following values for x from the preceding formula: 



a (in m.) = I 


10 


20 


30 


40 


5° 


75 


100 


X (in mm.)= i-20 


0.12 


0.06 


0.04 


0.03 


0.025 


0.016 


0.012 


a (in m.) = 200 


300 


400 


500 


700 


1000 


00 




X (in. mm.) =0.006 


0.004 


0.003 


0.003 


0.002 


O.OOI 


0.000 





The diameter x of the circle (or ellipse) is evidently quite 
small, and a constant focal distances may well be maintained for 
all phototopographical work without producing any appreciable 
error. 

In order to enable the observer to obtain good definition 
in the pictures of objects not very distant from the camera the 
Italian apparatus was devised with a movable objective and pro- 
vided with a metal scale (a. Fig. iii, Plate LIII) extending in 
the direction of the camera axis, which reads zero (or rather 240 
mm.) when the camera has been focused upon objects at infinite 
distance. The millimeter graduation of this scale, extending 
in the direction toward the sensitive plate of the camera, enables 
the observer to measure the focal length directly if the same 
had been changed at any time frbiii the principal focal distance 
( = 240 mm.). The objective cylinder M, Fig.' Hi, Plate LIII, 
may be inoved in the direction of the camera axis by revolving 
it within N, both tubes iV and M being connected by means 
of a screw, the rise of its thread being i millimeter. 

The circumference of N is divided into ten ecfual partsj and 
the position of the metal scale a, passing over this graduation, 
when the objective tube M is screwed into N, will indicate the 
tenths (and estimated' hundredths) of millimeters which it had 
been moved beyond the number of millimeters read off on scale a. 

The focal'length plays a very important r61e in all photo- 



2l8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

topographic work, and it is advisable to verify, at the beginning 
of operations, the reading of the metal scale, and if the principal 
focal length has been changed, the difference must be entered 
into the note-book, so that the proper correction may subsequently 
be applied. 

The distance of the point of view from the perspective plane, 
the position of the principal line, and the correct position of the 
horizon line can always be ascertained or rectified by instru- 
mental observations and computations, or graphically (if the 
picture plane has been exposed ui vertical plan or if its deviation 
from that position be known) as has been indicated, and as will 
be shown more fully later. 

It has been described how the optical axes of the telescope 
and of the camera are brought into two vertical and parallel 
planes. Both may be kept in this position and yet be revolved 
about the vertical axis of the instrument in order to successively 
expose the plates covering the entire panorama. The horizontal 
Umb of the theodolite is divided into 360° with subdivisions reading 
to 20', and by means of two verniers 30" may be read ofif. The 
vertical circle is provided with the same graduation and similar 
verniers. Thus the means are provided to ascertain the azi- 
muthal positions of the camera axis (the principal ray) for each 
perspective, or the means of " orientation " are thus provided 
for. The magnetic azimuth of the principalray of the perspec- 
tives (i.e., direction of optical axis for each exposure) or the hori- 
zontal angle which is included between said ray and any other 
Une passing through the station and some known point on the 
photograph (e.g., trigonometrical point) may readily be ascer- 
tained by observation. 

All perspectives that are to be used for mapping must be 
obtained from stations with known geographical positions. Gen- 
erally trigonometrical points are selected for the camera stations, 
but if points beyond these have to be occupied to better con- 
trol the topography, the elements needed (horizontal and ver- 
tical angles) to determine their positions with respect to Sur- 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 219 

rounding triangulation points may readily be observed with 
the theodolite before leaving the detached camera station. 

B. L. p. Paganini's new Phototheodolite (Model of 1894). 

The following description of Paganini's new phototheodolite 
has been extracted from his "Nuovi Appunti di Fototopagrafia," 
Roma, 1894. 

Paganini's new phototheodolite, model of 1890, differs from 
the one just described, although the general form and the dimen- 
sions of the camera-box, as well as the focal length of the lens, 
remain about the same as with the older model. The principal 
change rests in the omission of the eccentric telescope, which 
has been replaced by the centrally mounted camera, which may, 
at will of the observer, be converted into a surveying-telescope. 

The telescope which we generally find attached to surveying- 
instruments consists of a tube, slightly conical in shape, having a 
positive lens (or a system of convergent lenses) at one end, known 
as the "objective," which produces within the telescope a real 
and inverted image (the same as the camera-lens) of any object 
towards which this telescope may be directed. The other smaller 
end of the telescope-tube has a still smaller tube inserted into it 
which may be moved in the direction of the axis of the tube. 
This second tube also contains a system of convergent lenses — 
it is the so-called "ocular " lens set or "eyepiece " of the telescope 
— which serve to project an enlargement of the image — formed 
m the telescope — upon the retina of the observer's eye. 

In the image plane of the objective the so-called diaphragm is 
placed; it is a ring-shaped metal disc to one side of which a pair 
of cross-hairs is attached in such a way that the hairs (spider 
webs or Unes cut into the surface of a thin plano-paraUel glass 
plate) will coincide with the image plane. One hair is horizontal 
and the other vertical, their point of intersection falling in the 
optical axis of the telescope. 

A suitable eyepiece had only to be combined with the objec- 
tive of the older camera model to convert the camera into a 



220 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

telescope. The eyepiece of the camera tejescope, camera model 
of i8go, consists of a positive lens set, known in optics as Rams- 
den's ocular lens. 

The inner wall surfaces of the camera-box should be well 
blackened to avoid side reflections and a consequent dimness in 
the appearance of the cross-wires of the camera telescope. 

The camera proper consists of two parts, a truncated pyramid 
A, Figs 1 1 7-1 1 9, Plates LVI-LVIII, and a cylindrical attachment 
B containing the tube t. A second tube, placed v/ithin the tube 
t, may be moved in the direction of the optical axis by means of 
a screw the threads of which have a rise of i millimeter. By 
rotating this inner tube the lens may be brought nearer to or 
farther from the image plane, the lens remaining parallel to the 
image plane at any position that may be thus given it. 

A scale a, Fig, 117, Plate LVI, graduated to millimeters, is 
permanently attached to the tube t and it lies very close to the 
ring n, the circumference of which is divided into ten equal parts 
(this graduated ring n is soldered upon the cylinder u encasing the 
camera-lens). This scale a (extending in a direction parallel to 
the optical axis of the lens) has a mark coinciding with the index 
rim of the ring n, thus indicating the focal length of the camera- 
lens when focused upon objects at infinite distance. The miUi-, 
meter graduation of the scale a, extending from the zero mark 
towards the ground-glass, serves to ascertain the focal lengths 
for objects nearer the camera. The; graduation on the ring n 
serves to read one tenth of one revolution of the tube u, which is 
equal to an axial motion of the lens of q.i mm., hence the focal 
length for any object focused upon may be read to single miUi- 
meters on the scale a and to tenths of a milHmeter on the graduated 
ring n. 

The construction of this phototheodoHte is such that the optical 
axis of the camera-lens is always at right angles to the picture 
plane (the ground -glass surface or seiisitive fihn of the photo- 
graphic plate). The intersection of the optical axis and the 
picture plane (the so-called principal point of the perspective) is 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 221 

marked by the intersection P, Fig 113, Plate LV, of two very 
fine platinum wires, 00' and //', one horizontal and the other 
vertical. They are stretched across the back of the camera-box 
as close as possible to the picture plane. The buttons b, Figs. 117 
and 118, Plates LVI and LVII, serve to impart tension to the 
wires. The horizontal line 00' corresponds to the horizon line 
and the vertical line //' corresponds to the principal line of the 
perspective represented by the image on the ground-glass surface. 

Fig. 119, Plate LVIII, shows the rear view of this instrument, 
the ground-glass or focusing plate having been replaced by an 
opaque plate stiffened by a metal frame, which supports the 
Ramsden eyepiece in the center in such manner that its optical 
axis coincides with that of the camera-lens. The cross-wires 00' 
and //', at the rear of the camera-box, serve also for the astronom' 
ical telescope, into which the camera may be converted by attach- 
ing the opaque plate with central eyepiece as shown in Fig. 
119, Plate LVIII. 

The fitting of the eyepiece allows for axial motion to adjust 
its position with reference to the cross-wires to avoid parallax. 
The opaque plate supporting the eyepiece is composed of a thick 
cardboard impregnated with chemicals to harden its fibers and to 
render it impervious to moisture. The camera-box is made of the 
same material and both are strengthened by a frame and ribs of 
metal, as indicated in Figs. 117 and 118, Plates LVI and LVII. 
The cylindrical tube B is inclosed by a metal collar C which is 
held in position within the metal ring W by four screws R, R', S, S'. 
The ring W is connected with the frame gg' by means of two arms 
Ig and I'g', all being cast in one piece. The pivots q attached 
to the frame gg' serve as horizontal axis of rotation for the 
camera. 

This instrument is provided with a vertical circle, horizontal 
circle, verniers, reading-microscopes, levels, clamps, and tangent- 
screws, forming a complete transit with centrally located camera 
telescope. 

A cross-section of this instrument is illustrated in Fig. 120, 



222 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Plate LIX. The scale a, already described, is here placed on top 
of the tube u, to better illustrate its use. 
37 = uprights forming the supports of the horizontal axis of 
rotation for the camera telescope; 
/j= upper horizontal limb, or alidade, supplied with two verniers; 
il = lower limb, or horizontal circle, bearing the graduation; 
rr= tripod head, supporting the instrument by means of three 
leveling-screws (W); 
= casing for conical center; 

g'= central clamp-screw entering a ball which is supported by 
the hemispherical socbet w of the lower part of a. g' 
secures instrument to tripod-head and it guards against 
an accidental falling off of the instrument. 
The horizontal circle, having a diameter of 17 cm., is grad- 
uated into 20 minutes and suitable verniers are supplied to read 
horizontal angles to 30 seconds. 

The vertical circle, with a diameter of 10.5 cm., is graduated 
into 30 minutes and its verniers read to single minutes. 

The photographic plates are 18X24 cm., which is the same 
size as for the older instrument, model 1884. 

The objective lens, at first selected, was an aplanat of Stein- 
heil, and it had a focal length of 237.7 ™™- More recently, 
however, the Italian phototheodolites have been supplied with 
anastigmatic lenses of Zeiss. 

The column E, Figs. 11 7-1 19,- Plates LVI and LVIII, forming 
a prolongation of the lower arm I'g', is held in place by two 
counter screws m and m', which serve to hold the horizontal axis 
of rotation of the camera in a fixed position, preventing accidental 
changes that might otherwise take place during the execution of a 
set of panorama pictures. 

By unscrewing the nuts d', Fig. 120, Plate LIX, the tripod 
legs may be removed and they may then serve as '' alpenstocks " 
during the transportation of the instrument from station to sta- 
tion. The " camera telescope " may be Ufted out of the Y's and 
packed separately, the lower part of the instrument— the sub- 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 223 

Structure — is packed in another carrjdng-case, while the plate- 
holders and plates are transported in a third case. 

C L. P. Paganini's Photographic Azimuth Compass (Photo- 
graphic " Azimutale"). 

During the last years of Paganini's service as an officer in the 
Royal Navy of Italy, while in command of the cruiser " TripoUs," 
he was engaged upon work connected with making descriptions 
of the coast ("coast-pilot work"), and with hydrographic surveys 
for the construction of harbor and sailing charts. This work, 
undertaken for the production of better navigation guides, en- 
tailed a minute study of the approaches from the sea and a 
thorough reconnaissance of the coastal belt of topography. 

Landmarks available and useful for sailing-guides were to be 
accurately determined and plotted upon the hydrographic charts. 
Paganini was particularly instructed to obtain pictorial views of 
certain coast regions, showing the appearance of the coast when 
viewed from certain points off shore and giving the magnetic 
bearings to certain reference points shown on the views from the 
points marked on the charts whence the views were obtained. 
The point of view was determined either by means of the three- 
point problem, or it was indicated by means of the estimated 
distance from some well-defined landmark (lighthouse or other 
prominent object) giving also the magnetic bearing. 

The perspectives used to illustrate parts of the coasts, in order 
to facilitate the identification of the " landfall " by mariners 
when approaching the coast from the open sea, were pubhshed 
either on the charts or in so-called coast-pilot books. Formerly 
they were obtained by drawing a free-hand perspective from the 
deck of a vessel, including all prominent features and sailors' 
landmarks, particularly lighthouses, lone trees, prominent bluffs, 
and other characteristic features or marks. 

Such perspectives (Beautemps-Beaupr6 and Porro had a 
remarkable skill in producting accurate perspectives of such 



2 24 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

terrene views by offhand sketching) could not be made with 
mathematical precision, and very few draughtsmen have the gift 
to draw these perspectives under such conditions rapidly and of 
uniform scale. Their subsequent reduction to paper, under 
application of empirical and often arbitrary rules, based on sex- 
tant angles, magnetic bearings, and the mentioned defects in the 
sketches, had but a doubtful value, particularly if the vessel, 
during the time period which was consumed while these various 
observations were made, had gradually changed its position, due 
to winds and sea currents. 

Paganini, fully appreciating these difficulties, soon recognized 
the value of photography for obtaining such perspectives more 
readily and far more accurately, if a suitable photographic instru- 
ment could be constructed to be used on shipboard, the use of 
an ordinary camera, of course, being precluded. 

For several years Paganini made studies and experiments 
with the above object in view, particularly since the instantaneous 
photographic process had been perfected, and the photographic 
azimuth compass ("azimutale fotografico") is the direct result 
of his labors in this direction. It was devised to subserve the 
demands touched upon in the preceding paragraphs, and the 
instrument, described in Paganini's "De nuovi Appunti," has 
been made by Galileo in Florence. 

This instrument may serve to locate with accuracy lighthouses, 
buoys, and rocks awash, to obtain topo- and hydrographic views of 
the coast (for coast, harbor, and wharf surveys) for explorations 
and scientific expeditions, for the survey of unexplored coastal 
belts, for naval reconnaissance, for picturing naval displays and 
engagements, etc. It is furthermore adapted to determine the 
geographical latitude of a vessel's position by photographing the 
altitude of the sun above the sea horizon, including its magnetic 
bearing at the moment of the plate's exposure. From a negative 
showing the image of both the sun and the sea horizon the 
declination and the azimuth (magnetic) of the sun may be de- 
duced, and the time being known when the plate was exposed, 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 225 

the geographic position of the camera may be deduced from the 
plate. 

Paganini's photographic azimuth compass is shown, in a 
general way, in Figs. 130 to 133, Plates LXV to LXVIII. The 
photographic plates are exposed in this instrument with the long 
sides (24 cm.) horizontal and the short sides (18 cm.) vertical. 

The objective is very similar in arrangement to that described 
for the Italian phototheodolite, having a graduated scale to enable 
the observer to obtain the focal length directly. The two cross- 
wires, with their point of intersection in the optical axis of the lens, 
are secured in the image plane, the same as with the phototheo- 
dolite. 

The camera-box C is supported by two upright pieces S, shaped 
like an inverted U; at the top they are united by a horizontal plate 
Z, extending from the two camera sides around to the front of the 
camera, forming a horizontal connection in the shape of a horse- 
shoe. Three projections d, one at the front and two at the sides 
of the camera, serve to support the camera-box upon the horizontal 
frame Z by means of three pairs of counter screws v. In the 
sectional view of the instrument. Fig. 133, Plate LXVIII, r indicates; 
the vertical axis of rotation, gg the horizontal circle, B the azimuth 
or ship's compass, VV leveling-screws (supporting the horizontal 
circle on the heavy plate TT), Q a heavy weight (to lower the 
center of gravity of the apparatus and to increase the stability of 
the same when used on the deck of a rolling vessel), m is the 
handle of central screw (clamp) which passes through Q and enters 
a spherical nut attached below the horizontal circle, to allow for- 
lateral swing when leveling the camera with the foot screws F.. 

When this instrument is used on land the gimbal support is; 
replaced by a special surveying-tripod, the instrument resting on 
the latter by means of the three leveling-screws V. The illustra- 
tions show the instrument with the gimbal support the way it is 
^ used on shipboard. The three leveling-screws V rest upon a 
plate TT which is connected with the stout ring A by gimbals, 
the ring ^4 in turn being supported by four stout legs. The 



226 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

weight Q is sufficiently heavy to assure the vertical axis of rotation 
r, Fig. 133, Plate LXVIII, to remain always vertical. This 
apparatus is best adjusted and tested on shore, in order to adjust 
the horizontal thread 00' by means of the sea horizon. 

Below 'the under side of the camera-box (the latter is inclined 
about 30°) another smaller camera c is placed, close to the middle 
of the rear end, having a prism attachment by means of which a 
section of the compass graduation is reflected upon the lower edge 
of the photographic plate, the pictured graduation extending to 
both sides of the pictured principal Hne //', as indicated in Fig. 134, 
Plate LXVIII. The shutters of both cameras, C and c, are 
operated simultaneously by pressing the rubber bulb b, Fig. 132, 
Plate LXVII. The rubber tube attached to 'the bulb is forked, 
a separate branch leading to each pneumatic shutter, as indicated 
in Fig. 132. The optical axes of the cameras are at right angles 
to each other and, both are in the vertical plane conta,ining the 
principal line //'. 

The diameter of the dial compass passing through the zero 
mark of the graduation is identical with the magnetic meridian, 
and the compass-reading, designated by the graduation mark 
that is bisected by the prolonged vertical thread //' below the 
picture, represents the magnetic azimuth of the optical axis of 
the instrument at the moment of the exposure, or it will indicate 
the angle of orientation for the picture. The vertical frame E, 
Figs. 132 and 133, Plates LXVII and LXVIII, has a set of cross- 
wires with their point of intersection in the vertical plane which 
passes through the optical axis of the camera. A peep-hole 0, 
also situated in the vertical plane passing through the optical 
axis of the camera, is affixed to the upper horizontal limb, and 
with the cross- wires in E, it will enable the observer to direct the 
camera to any point that is to be bisected by the principal line 
when a plate is exposed. 

Also this instrument is provided with a Zeiss anastigmat lens 
of 250 mm. focal length. Eastmann's films or plates are used 
(18X24 cm.) and the horizon may be covered by eight plates, 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 227 

allowing a liberal marginal overlap, each plate covering an angle 
of about 50° horizontally. 

D. Photo grammetric Theodolite of Pro}. S. Finsterwalder. 

This phototheodoUte, devised by Prof. S. Finsterwalder after 
many years of practical work and experience, gained in his alpine 
surveys and studies of glacial motion, has been constructed by 
Max Ott (A. Ott), Kempten, Bavaria. In the pursuance of this 
work Prof. Finsterwalder early recognized the desirability of a 
surveying- camera of compact build, rigidly constructed in all 
its parts and yet having a minimum of weight. To avoid the 
transportation of a separate transit or theodolite for the trig- 
onometric location of the selected camera stations, he provided 
the surveying-camera with means for observing horizontal and 
vertical angles. 

This phototheodoUte is represented in Fig. 135, Pate LXIX, 
and the total weight of the outfit, 10 kgr., is distributed as follows r 
The instrument itself weighs 2.7 kgr., its carrying case 2.4 kgr., the 
tripod 1.7 kgr., one dozen leather receptacles, including twelve 
photographic plates 2.5 kgr., and the packing-case for the latter 
0.7 kgr. 

Prof. Finsterwalder has used both an anastigmat lens of Zeiss 
and a double anastigmat of Goerz, with a focal length of 150 mm. 
With this focus the lens will photograph perspectively correct a 
plate of 160X200 mm. The plates used are 120X160 mm., 
giving an effective horizontal field of 53°, enabling the observer 
to cover the entire panorama with seven plates. 

For the central or normal position of the objective this camera 
commands an effective vertical field of +20° and —20°, or 40° 
in all. Twenty degrees above or below the horizon of the camera 
station woiUd often be insufficient, particularly when working in 
mountainous tertene. It was deemed advisable, therefore, to 
mount the objective on a slide, permitting considerable- change 
in the vertical sense. Owing to this device objects subtending 



228 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

angles of depression up to 35° (together with a vertical angle of 
elevation of 5°) may still be photographed on the vertical plate. 

In extreme cases, when it should become desirable to photo- 
graph objects subtending angles of +35° and —35° (or 70° in all), 
Prof. Finsterwalder recommends the successive exposure of two 
plates, one with the maximum elevation and the other with the 
maximum depression of the lens. Inclined plate-pictures are 
thus not only avoided but the effective plate surface is utilized to 
the fullest extent and the weight of the glass to be carried is 
reduced to its minimum. 

In order to obtain uniformly accurate and trustworthy results 
with the relatively short focal length, maintaining a constant 
distance between the lens and the sensitized surface of the plates, 
the latter are not placed into plate -holders of the usual pattern 
(where the variable thickness of the glass plates would affect the 
so-called "constant" focal length), but they are pressed against 
a metal frame instead, which frame forms the back of the camera- 
box, an arrangement very similar to that of Capt. E. Deville's 
camera. To insert the plate into the camera use has been made 
of Dr. Neuhauss's leather plate-holders, formed like a sack, B, 
Fig. 13s, Plate LXIX. The inner edges of the metal frame are 
graduated in order to locate the principal and horizon lines upon 
the negatives. 

These leather sacks have metal slot devices facihtating the 
transfer of the plates from the sacks to the camera and vice versa. 
By hooking the mouth of the sack to the upper edge of the rear 
camera side and opening the slot while holding the bag in a 
vertical position, the plate is allowed to slip from the sack into 
the carrier. Springs attached to the rear of the camera-box 
serve to check the sliding plate and prevent a too sudden drop of 
the same into the metal carrier where the plate is to be exposed. 
These springs also press the plate into perfect contact with the 
metal frame at the back of the camera-box when once in 
position. 

By withdrawing the upper curved handle (Fig. 135, Plate 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 229 

LXIX) at the back of the camera the tension of the springs may 
be reduced (or their action may be released entirely), when the 
plate will glide into position for exposure. After exposure the 
lower slide is withdrawn and the tension of the springs is again 
reduced, when the plate will slip into the empty sack B, which 
had been hooked to the lower edge of the rear camera side as 
shown in Fig. 135, Plate LXIX. 

The eccentricity of the center of gravity, by applying the weight 
of the sack, including plate, to one side of the camera, does not 
affect the general adjustments of the instrument sufficiently to 
throw the photographic plate out of the vertical plane when the 
exposure is made. 

The camera is accurately balanced when no sack is attached, 
in which form it is used to measure the angles that may be 
required for locating the camera station, both in the geographical 
and vertical sense, with reference to the trigonometric signals 
in the vicinity. 

In order to use this instrument as a transit the back of the 
camera is supplied with an eyepiece E, Fig. 135, Plate LXIX, 
of a magnifying power of from 7 to 8, forming a centrally mounted 
telescope with the camera objective O. The eyepiece is supplied 
with a cap or shutter to exclude the light when the instrument is 
used for photographing. A diaphragm with the usual cross-hairs 
is also attached to the eyepiece. 

The camera-lens (or the objective of the camera telescope) 
being movable in the vertical sense within a range of 100 mm., 
all objects falling within a vertical range of ±17° may be bisected 
with the telescope. The definition of points to be bisected, when 
above or below the camera horizon, would become very poor if 
the eyepiece E were rigidly fixed in a horizontal position by 
means of the arms NN, Fig. 135, Plate LXIX. However, it may 
be revolved about a horizontal axis in such way that it will always 
be directed toward the center of the camera-lens. 

With the double anastigmat of Goerz, which produces a per- 
fectly flat picture, with neither spherical, chromatic, nor astig- 



230 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

matic distortion, a change in the focus of the eyepiece will 
rarely be necessary. 

Horizontal angles may be observed directly by means of a 
horizontal circle il of 120 mm. diameter, which is provided with 
two verniers reading to single minutes. Experimental tests 
made with this instrument have shown that horizontal angles 
between points of considerable difference in altitude may be 
observed within a limit of error of 0.4'. This instrument, there- 
fore, gives results sufficiently accurate to locate the camera stations 
trigonometrically with reference to surrounding fixed points of 
known positions, provided they are not too far distant to be 
defined with this low-power camera telescope. 

Vertical angles, however, cannot be measured directly; still, 
by means of the scale and vernier attached to the lens slide or 
front board of the camera, changes of the objective from its 
central or normal position — values directly proportional to the 
trigonometric tangents of the vertical angles — may be read to 
0.05 mm. The slide motion of the front board is accomplished 
by means of a rack and pinion, and experience has proven that 
vertical angles may be observed with this device within a limit of 
of error (converted into arc measure) of i'. 

The three rods designated by /j in Fig. 135, Plate PXIX, are 
100 mm. long and they serve to elevate the instrument support, 
together with the three leveling-screws S, sufi&ciently high above 
the tripod legs to allow full play for the leather plate-holders B, when 
they are placed in position to receive the exposed plate. The tri- 
pod legs may be folded to half their lengths. No ground glass 
being provided, a special finder has been devised correctly show- 
ing the field controlled by the plate (in both the vertical and 
horizontal sense) for any position of the camera-lens. (Zeitschrift 
fiir Instrumentenkunde, Oct., 1895.) 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 231 

E. Phototheodolite }or Precise Work by O. Ney. 

In the construction of this instrument, Figs. 136 and 137, Plates 
LXX and LXXI, it has been sought to fulfill the following 
requirements : 

First. The camera should be dimensioned for the exposure of 
plates sufficiently large to produce clearly defined perspec- 
tives. 
Second. The general disposition of weight and mass should be 
symmetrical. Both camera and telescope are to be mounted 
centrally. 
Third. The total weight of the instrument is to be reduced to 
the minimum consistent with rigidity and sufficient strength 
to assure stability and permanency of its adjustments when 
used in the field. The integral parts are to be formed 
to permit a free and easy manipulation of the instrument. 
Ney's instrument is composed of two distinct parts — the 
camera proper and the transit theodolite — which may be used 
successively and independently one of the other, but always upon 
the same tripod. The interchange between the two is readily 
accompUshed with accuracy and expediency. 

The principal advantages attached to this disposition of the 
component parts of Ney's phototheodolite may be cited as follows : 
First. The symmetrical and central mounting of the camera 
and transit telescope will insure permanency in the adjust- 
ments with consequent accuracy in the results. 
Second. By using the same tripod and horizontal circle T, 
Figs. 136 and 137, Plates LXX and LXXI, for both 
camera and transit their individual weight has been reduced 
to a minimum. 
Third. A possible disturbance of the adjustments of the instru- 
ment support D and tripod may be guarded against by hav- 
ing the plate inserted and the slide withdrawn before placing 
the camera- box in position on the upper alidade limb A, 
Figs. 136 and 137, Plates LXX and LXXI. 



232 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Accuracy and ease in the manipulation of this instrument 
have been assured by supplying all leveling- and clamp- 
screws with spherical ends which rest upon suitable plates in 
such a manner that a free play of motion will take place. These 
spherical terminations of the screws were originally devised by 
Reichel. 

Fig. 136, Plate LXX, illustrates the phototopographic camera 
mounted for use. Fig. 137, Plate LXXI, represents the transit 
with a striding compass B. 

D is the very rigid, yet essentially light, instrument support. 
The three arms supported by leveling screws are cast in one piece 
with the bearing for the conical pivot, which in turn is securely 
attached to the alidade T. The instrument support D, horizontal 
limb T, upper ahdade limb A, together with the skeleton tripod j 
are used in common for both transit and camera. 

A large circular level R is permanently secured to the center of 
the upper alidade limb A. 

Three hardened plates are inserted into the upper surface of A 
at S. One has a plane surface, the second has a conical cavity, and 
the third is provided with a V-shaped slot or groove ; they receive 
the spherical ends of the three screws which support the transit 
and the camera. 

The horizon hnes of both instruments may be adjusted and 
brought into the same horizontal plane by means of these sets of 
three screws each, which are attached to the base of both camera 
and transit. 

Either instrument may be securely bound to the common 
support A by turning the horseshoe-shaped clasps C, hinged to 
A, over the ends of the three screws and giving the levers E a 
quarter-turn. 

The transit telescope is arranged for stadia-reading (after 
Porro's method), having 100 as the common multiplier. The 
telescope-level is graduated to 20". 

The compass graduation reads to 30' and the horizontal circle 
reads either to 10" or 20", according to the size of the instrument. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 233 

The larger class instrument has plates 18X24 cm. and the small- 
sized camera has plates 13X18 cm. 

To avoid changes in the dimensions of the camera-box due to 
hygroscopical influences of the atmosphere it is constructed 
entirely of aluminum. The plate-holders and the movable carrier 
are made of mahogany impregnated with paraffine to render the 
wood' impervious against moisture. 

To avoid any possible change in the constant focal length, 
due to any uneven thickness of the plate-holders or of the photo- 
graphic glass plates, the carrier may be moved forward in the 
direction of the camera axis by means of the levers H, Fig. 136, 
Plate LXX, until the sensitized -film surface is brought into direct 
contact with a metal frame securely fastened to the walls of 
the camera. This frame has a centimeter graduation filed into 
its inner edges, and the distance of the rear surface of the frame 
from the nodal point of the camera-lens constitutes the constant 
focal length of the camera. 

The centimeter graduation on the inner edges of the metal 
frame, reproduced on the margins of the negatives, serves a double 
purpose. By its means the principal and the horizon line may 
be drawn on the face of each negative, and it also serves to ascertain 
whether the sensitized films, or the paper prints, have under- 
gone any change in dimension during the process of development. 
If distortion has taken place the amount of correction to be applied 
to the print may readily be found. 

The camera is provided with a pair of cross-levels to enable 
the observer to detect any change in its adjustment before expos- 
ing a plate. These levels have a graduation corresponding to 
20" of arc. 

When the instrument is in perfect adjustment, the picture 
plane will be vertical and the principal ray will be in the same 
horizontal plane as the optical axis of the horizontal telescope 
if the camera were replaced by the transit (without disturbing 
the position of the tripod and instrument support). 

When this camera theodolite is adjusted the vernier M, Fig. 136, 



234 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Plate LXX, will read zero for the normal position of the lens. 
The objective may, however, be elevated or depressed by 35 mm., 
and any change from its normal position may be read correctly 
within 0.1 mm. on the scale and vernier M. 

The pneumatic camera-shutter is arranged for either time 
or instantaneous exposures; a special device guards against 
the possibility of exposing a plate before it is brought into per- 
fect contact with the graduated metal frame. Neither can the 
plate-holder be withdravwi from the camera before the slide 
has been replaced nor as long as the plate is in contact with 
the graduated frame. (Zeitschrift fiir Instrumentenkunde, 1895, 
P- 55-) 

F. Phototheodolite of Dr. C. Koppe. 

Dr. Koppe, Professor at the Technical High School in Bruns- 
wick, Germany, is an ardent advocate of photogrammetry and 
he has done much toward popularizing photographic surveying 
in Germany. 

This phototheodolite. Fig. 138, Plate LXXII, has a centrally 
mounted camera K with the transit telescope T on one side and 
the vertical circle C on the other side. 

The horizontal axis has been widened between the wyes to form 
a conical ring R into which the camera K may be inserted. Four 
stout springs / press the camera securely against the ring surface, 
forming the collar of the conical ring. After insertion into the 
ring the camera is revolved, within the former, about its axis until 
the end of the screw h abuts against the stop d, when the prin- 
cipal line of the negative should be in vertical plane (the horizon 
line horizontal). 

The camera axis is parallel with the optical axis of the tele- 
scope T, both axes being in the same horizontal plane when 
they are level. This parallelism between the two axes is per- 
manent. 

The instrument will be in perfect equilibrium with the camera 
either attached or removed. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 235 

The horizontal axis of revolution may be adjusted by means 
of the striding level L, which, when required, may be replaced 
by a compass very similar to that shown in Fig. 137, Plate LXXI. 

Since the telescope (and camera) may be reversed in the wyes 
the error of coUimation and any index error of the vertical circle 
may readily be found or eliminated. 

Neither slides nor plate-holders are provided with this instru- 
ment, the plates being inserted directly into the carrier of the 
camera. This may be done in the field by aid of the packing- 
case. Fig. 139, Plate LXXII, specially devised, to serve as a dark- 
chamber. 

The case proper is made of wood with double doors, each 
door having a circular opening A, Fig. 139, Plate LXXII, filled 
in with a flexible Ught- and water-tight material, forming sleeves, 
in such a way that the hands of the operator may be thrust through 
an elastic opening in the center of the apron. The fabric closes 
tightly around the wrists, leaving the interior of the case in per- 
fect darkness and permitting free play of the hands in the 
interior L for manipulating the camera and plates within the 
case. 

The wooden box is incased in a tight-fitting sole-leather 
covering having two flaps S to protect the openings A against the 
admission of dust when the packing-case is transported on the 
back of the instrument-bearer. 

The entire instrument, except tripod, may be stowed away 
in the case for transportation and safe-keeping. The packing- 
box also contains two receptacles Ki and K2; one contains the 
unexposed plates and the other receives those that have been 
exposed during the day's work. 

When an exposed plate is to be exchanged, the camera C, 
Fig. 139, Plate LXXII, is placed into the packing-case, and doors, 
as well as the leather main flap, are securely fastened, the hands 
are inserted into the sleeves A, and the exposed plate P is removed 
from the camera and placed into its receptacle K^, closing the 
door T. A new plate g, taken from the box K2, is placed into 



236 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the camera and its back securely closed, when it will be ready 
for another exposure. 

The constant focal length of this camera is represented by 
the distance between the second nodal point of the lens and the 
rear surface of a graduated metal frame permanently secured 
to the walls of the camera. The irmer edges of this metal dia- 
phragm, or frame, bear a centimeter graduation; the middle 
graduation marks of the two horizontal sides locate the prin- 
cipal line, while the middle graduation marks of the vertical 
sides represent the termini, on the photographic perspectives, of 
the horizon line. 

The focal length, once determined, remains the same for all 
plates. This instrument has been manufactured by F. Rand- 
hagen in Hannover, Germany. 

The Topographic Bureau of Switzerland has used a photo- 
theodolite constructed after the model of Dr. Koppe's instrument. 
The experience in Switzerland, however, seems to have decided 
the Bureau of Topography not to replace the plane table by the 
phototheodolite for general topographic surveys executed by that 
bureau. 

Dr. C. Koppe's new Instrument and Method for Observing Horizon- 
tal and Vertical Angles directly on the Photographic Negative. 

We have seen, on page 125, that the iconometric plotting of 
vertical and horizontal changes in the terrene were based upon 
direct measurements of the coordinates of pictured points. Dr. 
Koppe, in his recently pubUshed pamphlet on photogrammetry, 
with particular reference to cloud photography, has given a new 
method having many advantages for particular kinds of work. 

He inserts the negative into the camera in the exact position 
previously occupied by the plate during exposure and illuminates 
the same from the rear sufficiently to bring out all the details of 
the negative. With a theodolite telescope directed through the 
camera-lens he now observes the vertical and horizontal angles by 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 237 

bisecting the particulair points on the negative in the same manner 
as the surveyor uses a transit for observing in the field. 

Light-rays reaching the camera-lens from a distant point in 
parallel directions are concentrated and directed to one particular 
point on the photographic plate — the image point. With Dr. 
Koppe's device the same phenomenon takes place only in inverse 
order as the light-rays now emanate from the illuminated negative. 
Light-rays coming from any point of the negative are refracted by 
the camera-lens whence they enter the telescope of the theodoUte 
in parallel directions. If the telescope had been focused for 
infinite distance the pictured point observed upon will appear 
sharply bisected by the cross-webs in the telescope, and if the 
camera is securely fixed in position and the telescope is now changed 
to bisect another pictured point, the angle included between the 
directions to the two pictured points successively bisected will 
be identical with the angle included between the lines of direction 
drawn from the first nodal point of the lens at the original camera 
station in the field to the corresponding two points in nature. 
We can, therefore, obtain the values for the vertical and horizontal 
angles by reading the corresponding verniers of the theodolite in 
both positions of the telescope when bisecting the pictured points. 

In the practical application of this method two cases are con- 
sidered. The camera is either stationary and the observing tele- 
scope adjustable in position, or the. telescope is stationary and 
the camera is adjustable. Dr. Koppe has experimented with both 
types of instruments. 

If we insert the negative in the camera, giving it the same 
position which the plate had during exposure, and if we level the 
instrument, giving it likewise the same position which it had 
when the plate was exposed in vertical plane, and finally, if we 
adjust the observing telescope of the transit to bring its optical 
axis in line with the horizontal axis of the camera, we can observe 
horizontal and vertical angles by bisecting the pictured points and 
noting the vernier readings. 

The main difficulty in the practical solution of this problem 



238 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

was the necessity of having to give the observing telescope a 
decidedly eccentric position to enable the observer to bisect 
pictured points at any place of the illuminated negative. The 
construction of this instrument, therefore, is more cumbersome 
than the ordinary theodolite or transit with centrally mounted 
telescope. 

Dr. Koppe declares the .degree of accuracy attainable in the 
application of this method to be the same as when the corresponding 
angles were measured in the field with an instrument of equal 
size and power. This assertion is based on practical tests and 
experimental observations made by Dr. Koppe in connection with 
the prehminary survey for the location of the railroad over the 
Jungfrau in the Alps. 

The principal advantages of this method, compared with 
angular measurements made directly in the field, may be found in 
the reduction of the field-work, in the possibility of measuring 
angles between objects in motion (using instantaneous plates for 
this purpose), in locating points of an evanescent character, like 
prominent features of distant mountains which may be liable to 
sudden disappearance under freshly fallen snow, or which may 
be visible only for short intervals, due to the frequent and sudden 
formation of "cloud-caps" or "hoods," etc. 

An advantage of Dr. Koppe's method against the general 
method of measuring the coordinates of pictured points rests in 
the fact that correct values for the horizontal and vertical angles 
may be obtained from negatives that are not geometrically true 
perspectives, owing to distortion produced by imperfect lenses or 
lens combinations, as the points of the illuminated negatives will 
emanate Ught-rays from the first nodal point of the objective in 
directions identical with tliipse in which they originally arrived 
there when the plate was exposed. The instrument used by 
Dr. Koppe in his experimental tests was manufactured by Oskar 
Giinther, in Braunschweig, after plans and specifications furnished 
by Dr. Koppe. It is illustrated in Figs. 140-142, Plates LXXIII 
and LXXIV. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 239 

The phototheodolite is essentially the same as has been de- 
scribed with a horizontal circle of 14.7 cm. diameter and vertical 
circle of 12 cm. Aperture of the eccentric telescope is 27 mm., 
with a focal length of 20 cm. and a Ramsden eyepiece magnifying 
twenty times. The verniers read to single minutes, admitting 
15 seconds to be estimated. 

The telescope r. Fig. 140, Plate LXXIII, used for angulation, 
in both the vertical and horizontal sense, upon pictured points 
has an aperture of 18 mm., a focal length of 8 cm., and its 
Ramsden eyepiece magnifies five times. 

The focal length of the camera objective (double anastigmat 
of Goerz, series III, No. i) is 144.9 ™™- 

Fig. 140, Plate LXXIII, shows the phototheodolite com- 
bined with the telescope r ready for angulation upon the pictured 
points of the illuminated negative in camera Q. 

Fig. 141, Plate LXXIV, represents the various attachments 
required to convert the phototheodolite, as used in the field, into 
the form shown in Fig. 140, Plate LXXIII, as used for the angula- 
tion in the office. These attachments, of course, need not to be 
taken into the field. 

For the use in office glass positives are preferably made by 
contact printing from the negatives, as it facilitates the identifica- 
tion of points for the " angulation on pictured points " if these have 
the same appearance regarding light and shadow as the natural 
objects which they represent. 

To identify corresponding points on two or more positives the 
plates are placed side by side on a frame with a reflector below 
them. Points selected for angulation are marked by fine " pricks " 
made with a needle. Points of reference that have been observed 
in the field are marked with red ink, giving identical points the- 
same designation, and points to be determined by angulation in; 
the office are marked with blue ink. A check for the correct 
identification of these points may be obtained in the usual manner 
by determining their elevations from two or more plates, using, of 
course, in this case the vertical angles obtained by angulation upon 



240 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the pictured points with the telescope r, Fig. 140, Plate LXXIII, 
in the office. 

The "spring frame" P, Fig. 141, Plate LXXIV, serves to 
give the various positive plates the positions in the camera corre- 
sponding with the positions of the original plates during exposure. 
This is readily accomplished by viewing the plate through the 
objective of the camera and adjusting the plate, held against the 
graduated frame at the back of the camera by means of the spring 
frame P, in such a way that the pictured graduation marks coincide 
with their corresponding marks on the graduated frame which is 
permanently fixed in the image plane of the camera. 

From the phototheodolites as used in the field we now 
remove the eccentric telescope, including its vertical circle and 
circular camera support, from the wyes and replace these with 
the telescope ^-and vertical circle B (Figs. 140 and 141, Plates 
LXXIII and LXXIV). The secondary camera support T-t, 
Fig. 141, Plate LXXIV, is then secured to the theodolite by 
means of the clamp-screws SS, in the manner shown in Fig. 140, 
Plate LXXIII, and the camera Q, with positive plate in adjusted 
position, is inserted into the circular camera support K (Figs. 140 
and 141, Plates LXXIII and LXXIV) and adjusted in posi- 
tion so that the first nodal point coincides with the point of inter- 
section of the horizontal and vertical axes of rotation of the tele- 
scope r. 

The positive plate, if well illuminated by diffused light, will 
now emanate rays from the marked points in the same direction, 
beyond the objective of the camera, as the incident rays origi- 
nally emanating from the corresponding points in nature at 
the time of the plate's exposure. 

We may now proceed to measure the horizontal and vertical 
angles of the marked points of the plate with the telescope r, 
provided the camera has the same inclination which it had when 
the original plate was exposed. This inclination may be estab- 
lished by clamping the telescope r in position when the verniers 
of the adjusted vertical circle B, Fig. 140, Plate LXXIII, read 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 241 

the same as the recorded mean value for the inclination of the 
camera in the field for the particular plate under observation, 
and tipping the camera, first roughly by means of the support t, 
Figs. 140 and 141, Plates LXXIII and LXXIV, then with the 
slow-motion screw h, Fig. 140, Plate LXXIII, and Fig. 141, Plate 
LXXIV, until the principal point of the photographic perspec- 
tive is bisected by the cross- webs of the .clamped telescope r, 
which is then released for the angulation of the marked points 
on the positive plate. 

Excepting the angulation upon the triangulation and refer- 
ence points in the field, the vertical and horizontal angles to any 
number of pictured points may be observed in the office by this 
ingenious device, by means of which the greater part of the field- 
work, when using tachymetric methods, may be transferred 
to the office, making such detailed observations independent of 
the length of the field season and of the vicissitudes of climate 
and weather. 

The computations are the same whether such observations 
are made in the field or in the office, both methods giving prac- 
tically the same results with instruments of this cl^ss. 

With Dr. Koppe's instrument the angular values were obtained 
in both cases within a maximum error of one minute if the 
bisected points were not farther away than 3000 m. 

H. Phoiotheodoliie Devised by V. Pollack; Manufactured by 
R. Lechner in Vienna, Austria. 

With this instrument the camera C, Fig. 143, Plate LXXV, 
is centrally located and mounted above the horizontal circle. 
The telescope F and the vertical circle V are attached to one 
side of the camera, but counterbalanced by the weight G. In 
order to reduce the weight as much as possible aluminum has 
been used extensively in the construction of this apparatus. 
For instance the upright T is made entirely of that metal. 

This instrument has been manufactured in two sizes; the 
horizontal circle of the smaller one is graduated to 30' and the 



242 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

verniers read to i', while the larger one has a circle graduated 
to 20' and its verniers read to 20". The telescope F is mounted 
somewhat like that of the Danish plane-table alidade. 

The adjustment of the horizontal axis of revolution of the 
telescope F is accomplished by means of a special striding level. 
Clamps and slow-motion screws are provided for both the hori- 
zontal and the vertical circles. The telescope has a focal length 
of 27 cm. and an aperture of 31 mm. with a magnifying power 
of 9 to 18 diameters. The telescope .is arranged for stadia- read- 
ing, and it has 100 as the constant multiplier. The telescope- 
level L is graduated either to 10" or 20". The vertical circle 
is graduated to 20' and its two verniers read to 20". The 
camera proper is made of aluminum and it is provided 
with a Zeiss anastigmat. By means of the rack and pinion, z, 
the lens may be elevated or depressed by either 30 or 50 nmi., 
according to the size of the instrument. The scale /, together 
with the vernier n, serves to measure the vertical deviation of 
the lens from its normal position. Also this camera is provided 
with a metal frame the inner edges of which have either a 
centimeter or a 5-miUimeter graduation, which is reproduced 
upon the negatives. This graduation serves not only to locate 
the horizon and the principal line upon the photographic per- 
spectives, but it also gives ready means for discovering any dis- 
tortion that may arise in the perspective, due to the wet process 
of development. The graduated metal diaphragm or frame is 
brought into direct contact with the sensitive surface of the film 
by a simple mechanical contrivance, in such a way that the 
focal length remains constant for all negatives, even if the 
plate-holders or plates should vary a little in thickness. 



I. Phototheodoliie Devised by Pollack and Hajjerl. 

This phototheodolite is shown in Fig. 144, Plate LXXV, 
It has been used under the Imperial General Directory of 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 243 

Austrian State Railroads, and it was placed on exhibition during 
the Ninth Convention of German Geographers in Vienna 
in i8gi. This camera has no graduated metal frame, its horizon 
and principal line being located by means of a set of four vanes 
or index marks, which are pressed against the sensitized film 
of the photographic plate by means of a revolving button, a 
device similar to that mentioned in connection with the older 
pattern of the U. S. Coast and Geodetic Survey camera. The 
instrument is leveled by means of two leveling-screws s, coun- 
teracted by two pivots /, Fig. 144, Plate LXXV, which are held 
in position by two spiral springs. 

K. R. Lechner's Photo grammeter. 

R. Lechner's photo grammeter is shown in Fig. 145, Plate LXXVI. 
(Lechner also manufactures Pollack's, Werner's, and Huebl's in- 
struments.) 

It is a rectangular metal camera, C, of constant focal length, 
centrally mounted upon a graduated horizontal circle, K. Two 
spirit-levels /, attached to the upper limb of the horizontal circle 
(which is graduated into degrees), and three leveling-screws 5 
serve to adjust the position of the instrument. Bi is the vertical 
axis of rotation for camera and horizontal circle. The tripod is 
set up approximately level, the circular level x being provided 
for that purpose. 

The camera is connected with the upper limb of the horizontal 
circle by four screws. Two of these are in the direction of the 
optical axis and serve to adjust the image plate into vertical plane 
(the levels / reading zero), the other two are situated in a Une at 
right angles to the direction of the optical axis; they serve to 
adjust the horizon line into horizontal plane. 

The objective, O, is a Zeiss anastigmat //18, and it may be 
elevated or depressed by means of a rack and pinion, such dis- 
placement being read on the scale, t, with vernier, n. 

A metal frame with inner edges graduated in centimeters is 



244 PHOTOTOPOGRAPHIC METHODS AND INSTEUMENTS. 

also provided for this camera, and a special mechanical device 
serves to press the plate (in the plate-holder after the slide had 
been withdrawn) against the rear surface of this frame so that 
the centimeter scale is impressed on the margin of the negative or 
photograph, thus providing the means to locate the horizon and 
principal Hnes upon the perspective and also to eliminate any error 
due either to a possible faulty or imperfect registration of the 
plate-holder or due to distortion in the photograph (paper print). 

In the middle of the back-board of the camera is an eyepiece 
with cross-threads, very similar in arrangement to that described 
for Paganini's new phototheodolite, forming a telescope with the 
object-glass of the camera. In this case, however, the cross- 
threads are attached to the eyepiece and their intersection coin- 
cides with the principal point of the perspective. 

A dial compass, B, is attached to the upper face of the camera- 
box, a being the catch to clamp the needle, or dial, when not in 
use. 

L. Phototheodolite of Col. A. Laussedat (new Model). 

Col. A. Laussedat' s latest phototheodolite, manufactured by 
E. Ducretet and L. Lejeune, Paris, France, is shown in Figs. 146 
and 147, Plate LXXVI. Both transit telescope and camera are 
centrally mounted, the latter above the former. The camera may 
also be separated from the transit, and by means of a special pivot 
or spindle s', Fig. 147, Plate LXXVI, it can then be mounted 
upon the same tripod. The transit may be used for trigonometric 
observations aftei: removal of the camera. Fig. 146, Plate LXXVI, 
represents the corftplete instrument. 

5S5 are the three leveling-screws and ci is the central clamp; 

C is the camera and B is the magazine for fifteen plates ; 

O is the objective of the camera (it is a rectilinear wide angle lens 

of 75 millimeters focal length); 
H is the sliding front plate provided with pinion and rack R to 
elevate or depress the lens; 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 245 

7 is a finder to show the extent of the field covered by the photo- 
graphic plate, although a focusing-glass is also provided; 
L is the transit telescope provided with stadia wires; 
Ce is the vertical circle graduated to 30'; 
MM are the uprights supporting the horizontal axis of revolution 

of the transit telescope and also supporting the camera; 
A is the horizontal circle graduated iftto 30' ; its clamp and slow- 
motion screw are shown at P'; 
N is the adjustable level, and 

X> is a long compass, with slow-motion screw and clamp at P, 
to read the magnetic azimuth on the horizontal circle A. 
Several loaded magazines, each containing 15 plates, may be 
carried with the instrument. The plates may be changed in full 
daylight without removing the camera. The plates are 6 J X 9 cen- 
timeters. Enlarged prints are used for the iconometric plotting. 
Six plates forming a panorama cover the entire horizon. The 
lens is provided with an iris-shutter, and it may be focused for 
short distances or infinity by turning a lever pver a scale, show- 
ing the distances in meters, attached to the front board H, Fig. 
146, Plate LXXVI. In Fig. 147, Plate LXXVI, the mstru- 
ment (camera) is represented with the magazine B removed and 
replaced by the groimd glass G. 

The entire outfit, excepting the tripod, which is carried sepa- 
rately, may be transported in a carrying-case (with shoulder- 
straps) 39 X 28 X 1 7 centimeters. The weight of the carrying- case, 
including instrument (complete), one magazine, and fifteen plates 
amounts to 8 kilogrammes. 

M. Phototheodolite of Starke and Kammerer. 

This instrument, represented in Fig. 148, Plate LXXVII, 
somewhat resembles in construction the phototheodolite of Prof. 
Finsterwalder; neither has a vertical circle and both have a 
" camera telescope." 

The camera is mounted on the horizontal circle like a theo- 
dolite. An ordinary skeleton tripod supports the three leveling- 



246 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

screws 5 of the horizontal circle, Fig. 148, Plate LXXVII, and 
a central clamp-screw P with spiral spring connects the tripod 
with the instrument proper. 

H is the horizontal circle, graduated to 20'; its two verniers 
with microscopes L read to single minutes. • 

The vertical axis, terminating in three horizontal arms Bi, B^, 
and B3, may be adjusted' by means of the leveling-screws 5 and 
the cross-levels h and h- The plate D, forming the support 
for the cross-levels, is firmly attached to the arm B2. 

£ = upper clamp-screw; 
Af = upper tangent-screw for slow motion; 
i<'i, ^2) and i^3= three leveling-screws supporting the camera- 
box; they rest in the grooves of the three 
arms Bi, B2, and B3; 
^3 and ^4= cross-levels attached to the camera, as shown 
in Fig. 143, Plate LXXVIII, which represents 
the camera-box viewed from above. 
These cross-levels, together with the screws Fi, F2, and F3, 
serve to adjust the photographic plate into vertical plane. 

5= movable front board, or objective slide; 
Q= handle to facilitate mounting of the camera; 
2^1= head of pinion to elevate or lower the camera-lens, the 

slide 5 having a corresponding rack for that purpose; 
2^2= differential pinion for slow motion; 
JT = clamp-screw to secure the slide 5 in a given position; 
m (Fig. 148, Plate LXXVII) = millimeter scale to measure 

the vertical change of the lens from its normal or 

central position, the vernier n permitting such change 

to be read to 0.05 mm. 

The camera may be securely fastened to the vertical axis 
of the horizontal circle by manipulating the central clamp-screw 
from the interior of the camera-box. 

When the instrument is in adjustment the zero mark of the 
vernier n will coincide with the 70 mark of the scale m, and the 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 247 

lens will then be in its central or normal position. The slide 5 
may be moved 70 mm. up or down; from 70 to 140 the lens will 
be above the normal position. 

The lens of this camera is a Zeiss anastigmat, //18, with 
a focal length of about 212 mm. 

The camera -lens is suitable for phototopographic purposes 
if the horizontal change in the distance between its second nodal 
point and the image plane does not exceed 

0.09 o.ii 0.15 0.22 o . 45 mm. for distances of 

500 400 300 200 100 m. 

Hence, focusing may be dispensed with for general photo- 
topographic purposes; still, in order that this camera, for special 
purposes, may also produce sharp and well-defined pictures of 
objects comparatively close to the camera, its lens has been 
mounted to admit a longitudinal motion, in the direction of 
the optical axis, , within a range of 2 mm., thus giving the 
means to focus objects that are only 23 m. distant from the 
camera. 

The external tube of the lens mount has a helical groove, 
or slot, in which a small metal block, t, Fig. 149, Plate LXXVIII, 
provided with an index mark, may travel. This block t is attached 
to the inner tube of the lens mount, and a screw r, Fig. 149, 
Plate LXXVIII, at one end of the slot serves to clamp the two 
tubes together when the focal length is to be maintained con- 
stant for any length of time. 

Loosening the screw r and revolving the outer tube from left 
to right will shorten the focal length, and when the block t has 
passed from one end of the slot to the other, the focal length 
will have suffered a change in length of 2 mm. 

The two positions of the index mark on the block t for these 
extreme limits are marked on the outer side o and 2, Fig. 149, 
Plate LXXVIII; the interval is divided into twenty equal parts, 
one division corresponding with an axial motion of the camera- 
lens of 0.1 mm. A metal frame is attached in the rear to the 



248 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

camera sides and the posterior surface of this frame coincides 
with the image plane. The inner edges of this metal frame 
are provided with a centimeter graduation, the middle marks 
of the two horizontal sides indicate the position of the principal 
line, while the middle notches of the two vertical sides mark 
the position of the horizon line. When the camera is adjusted 
these lines will intersect each other in the principal point of the 
photographic perspective. The inner opening of the metal 
frame is 17.8 and 22.8 cm., which, of course, is also the size of 
the pictures. 

The two frames / and II, shown in Figs. 150 and 151, Plate 
LXXVIII, give the means to make a light-tight connection 
between the single plate-holders (or ground glass) and the camera- 
box. The short bellows W, connecting frames / and II, will 
admit the frame // to be moved toward or from the frame /, 
which is securely attached to the camera-box. Each one of these 
frames I and II is provided with two hooks; frame / has an upper 
hook hu Figs. 148 and 149, Plates LXXVII and LXXVIII, 
and a similar hook near the lower comer diagonally opposite hi. 
The hook h2, Fig. 149, Plate LXXVIII, is attached to the upper 
comer of frame II, opposite hook hi, and frame II has a similar 
lower hook diagonally opposite /f2 and directly below hook hi. 

Fig. 151, Plate LXXVIII, represents the section of a rear 
comer of the camera-box, showing the ground-glass-plate attach- 
ment V (it also has the eyepiece forming a telescope with the 
camera-lens. Fig. 152, Plate LXXIX). 

Frame II is secured to frame / by means of tte upper left 
hook h^ and the lower right hook. The ground-glass frame V 
is supported by the screws Zi and Z^, Figs. 151 and 152, Plates 
LXXVIII and LXXIX, the points of which rest upon the metal 
plates Tt, Figs. 149 and 151, Plate LXXVIII, permanently 
attached to the fixed frame I. The face of the ground glass G, 
Fig. 151, Plate LXXVIII, is brought into contact with the rear 
surface of the graduated frame R by means of the upper right 
and lower left hooks. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 249 

The position of the optical axis of the eyepiece may be adjusted 
vertically by turning the screws Z\ and Z-i until the line of col- 
limation of the eyepiece and camera-lens coincide in the horizon 
plane, the camera-lens being in its normal position, the zero 
mark of vernier n coinciding with the 70 mark of the scale m. 
Fig. 148, Plate LXXVII. In this position points may be observed 
through the eyepiece of the ground-glass attachment. When the 
camera-lens has been moved some distance up or down (away 
from its normal position), however, the eyepiece can no longer 
be used with its line of coUimation in a horizontal position, and 
the clamps, or stops, Px and p2. Fig. 152, Plate LXXIX, are 
unfastened and the eyepiece is tilted up or down — it rotates about 
a horizontal axis x^^x^. Fig. 152, Plate LXXIX — until its optical 
axis is directed to the center of the object-glass. The image of 
the point to be bisected will then appear well defined. 

The circular openings p, shown in the ground-glass attachment. 
Fig. 152, Plate LXXIX, serve to examine the middle notches (of 
the inner edges of the graduated metal frame i?) which define the 
termini of horizon and principal line of the perspective. The 
openings p give the means to test the positions of those Unes with 
reference to the middle notches; both should coincide. The outer 
-wooden frame Y, Fig. 151, Plate LXXVIII, of the ground-glass 
attachment is strengthened by- two metal diagonals D joined into 
a ring at their intersection, which ring supports the eyepiece, 
revolvable about x^x^ as axis. 

Each holder contains but one plate, and Fig. 150, Plate 
LXXVIII, shows a section through the upper rear part of the 
camera with a plate-holder K in position. 

P= dry-plate; 

/= springs supporting the dry-plate at its four comers; 

G= hard-rubber sUde, which is completely withdrawn when a 
plate is to be exposed; 

i?= graduated metal frame permanently secured to the rear 
of the camera-box; 

C= section of camera-box wall. 



2 so PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

To attach a plate-holder K to the camera, Fig. 150, Plate 
LXXVIII, frame II is set free from I and K is hung to frame // 
by means of the bent plate I (permanently attached to K) when the 
beveled projecting frame or edge of K closes into the corresponding 
rebate of frame II, producing a light-tight connection. K is 
secured to II with the upper left and lower right hooks, giving 
it the position shown in Fig. 150, Plate LXXVIII. After the 
hard-rubber sHde G has been withdrawn, the second pair of hooks, 
upper right and lower left, are tightened to draw the holder K 
forward until the sensitive-film surface is brought into contact 
with the graduated metal frame R (at the back of the camera), the 
springs / serving as a cushion to insure a perfect contact without 
straining the glass plate P. The lens is now uncapped, the 
exposure made, and the holder is withdrawn by repeating the 
same operations in inverse order: unfastening the pair of hooks, 
upper right and lower left, inserting the slide G, and drawing back 
the two last hooks, lower right and upper left. 

N. Capt. von HubVs Plane-table Photogr ammeter. 

This instrument, manufactured by R. Lechner in Vienna, has 
been described in Lechner's " Mittheilungen aus dem Gebiete der 
Photographic und Kartographie," Wilhelm Miiller, Graben 31, 
Wien. It is the result of Capt. Hiibl's endeavors to reduce the 
weight and costs of an effective photographic-surveying instru- 
ment, to be easily adjusted and manipulated and to subserve 
topographical purposes. 

As the final result, generally aimed at in topography, reduces 
itself to the graphical representation of the terrene, Capt. Hiibl 
combined the surveying-camera with a plane table by means of 
which the directions needed for the orientation of the picture 
traces, as well as those required for the location of the camera 
station, may be plotted directly in the field. It is supposed that 
a sufi&cient number of triangulation points had been provided 
to locate the camera stations by resecting upon signals, of which at 
least three should be visible from every camera station. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 251 

■ For this purpose the camera top M (21X21 cm.) supports 
the plane table sheet, which is securely held in position by four 
metal comer clamps n, Fig. 153, Plate LXXIX. 

C = camera-box (of constant focal length) made of aluminum; 
r= graduated horizontal circle with clamp-screw. It enables 
the observer to turn the camera in azimuth by an equal 
amoimt (panorama section) after each successive ex- 
posure; 
yF=graduated metal frame; 

XX = correction screws for adjusting VV to bring the principal 
point into the optical axis of the camera-lens; 
J = rubber bulb for operating the pneumatic shutter of the 

camera; 
/=head of pinion, which serves to elevate or depress the 
camera-lens. Any such change from its normal posi- 
tion may be read off on a scale with vernier, secured 
at one side of the camera-lens; 
d= spirit-level. Two are provided (at right angles) for 
adjusting plane table M into horizontal plane (the 
photographic plate will then be in vertical plane); 
2?= movable plate -carrier; 
ZZ= lever to move the plate-carrier toward the lens until the 
sensitive-film surface of photograpic plate is brought 
into direct contact with the graduated metal frame VV; 
ir= plane-table alidade with vertical circle, arranged for 

• stadia-reading; 
Z= pivot vertically above second nodal point of camera-lens. 
The lever h serves to locate the principal point /, Fig. 154, 
Plate LXXIX; when the edge of the alidade ruler LL abuts 
against the upturned lever h the principal ray zf, bisecting the 
angle ezg, Fig. 154, Plate LXXIX, may be drawn upon the plane- 
table sheet. 

Fig. 154, Plate LXXIX, represents the plane-table top abed; 
it has two pivots z and z' about which the alidade ruler LL may 
be revolved. 



252 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

z/ = constant focal length of camera; 
e^= horizontal projection of the picture trace; 
€zg= horizontal angle commanded by each plate = panorama 
section. 

At e and g are two stops (corresponding to the ends of the 
photographic field) representing the ends of the picture trace 
on the plane-table sheet. 

By placing the alidade ruler LL upon the pivot z the hori- 
zontal projections of lines of direction, emanating from z as a 
center, to such points of the landscape which serve to orient the 
picture (so-called reference points) may be drawn upon the 
paper within the sector limits ezg. The point z, marked on 
the sheet, may be regarded as the plotted station point. The 
central pivot z' serves as vertical axis of rotation for the alidade 
ruler LL when the horizontal directions to known points (signals 
over trigonometric stations, visible from the camera station) are 
to be plotted to locate the position of the camera station. 
Point z', transferred to the plane-table sheet, is regarded as the 
plotted station point. 

z/ or z'\ is the trace of the principal plane upon the horizontal 
projection plane. 

The plane table M with alidade K serves to locate the camera 
stations in both the vertical and horizontal sense; it may also 
be utilized for the location of tertiary points with the stadia- 
rod and for sketching details in the neighborhood of the sta- 
tion. The horizon line and principal line may be located upon 
the perspectives by means of the centimeter graduation of YY, 
or two very fine wires may be attached to the corresponding 
graduation marks. 

The instrument rests upon the three ends of the leveling- 
screws S, Fig. 153, Plate LXXIX, which fit into slots at the bot- 
tom of the camera-box, the latter being firmly united with the 
tripod by means of a central screw with spiral spring. 

This photographic plane table is well suited for topographic 
reconnaissance surveys. The results obtained with it may not be 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 253 

as precise as those obtainable with the more complicated and 
refined phototlieodolites; still, it is more readily transported, 
is very easily manipulated, and its adjustments are not subject 
to frequent disturbance. The instrument is compact, well con- 
ceived, and excellently executed. 

The size of the photographic plate is 12X16 cm., giving an 
effective picture, within the graduated margin, of 10X14 cm. 

The cube-shaped camera weighs 3.5 kgr. 

The packing-case (knapsack), including the entire outfit and 
a stout tripod (with 3 folding legs), weighs only 11.5 kgr. 

The cost of this outfit in Vienna is 400 florins. 

O. Phototheodolite {" PhototachSonihtre") Devised by J. and 

H. Vdlot. 

In 1893, Joseph and Henry Vallot entered into correspond- 
ence with Col. Laussedat with reference to the planning of a 
phototheodolite, to be used in the topographic survey of the 
Mont Blanc mountain group. The first instrument made for 
this purpose did not give results with the anticipated and desired 
degree of accuracy; its adjustments were foimd too unstable for 
transportation in rough mountains. 

Owing to the defects in this instrument three himdred plates 
obtained in 1893 were discarded and a new phototheodolite 
with a constant focal length and with vertically exposed plates 
was devised. J. VaUot designed the geodetic part of the instru- 
ment and H. Vallot plarmed the camera. The manufacture was 
intrusted to Brasset Freres, who succeeded in furnishing an in- 
strument of great stability, small weight, and general excellence. 
It is shown in Figs. 155 and 156, Plates LXXX and LXXXI. 
The latter illustrates the theodoUte (" tacheometre ") and the 
former represents the surveying- camera. Both are mounted 
upon the same tripod and one instrument support with hori- 
zontal circle R answers for both. The following desiderata 
have been fulfilled by the makers of this instrument: 



254 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.. 

First. In order to give the least resistance to the wind, to 
facilitate transportation, and to reduce the price the size 
of the instrument has been reduced to a minimum com- 
patible with efficiency. 

Second. To avoid vibration of the plate during exposure the 
component parts of this instrument have been assembled 
and joined together with the utmost rigidity. 

Third. The instrument is well balanced to avoid strains in 
the instrument itself and to give the results obtainable 
with it the greatest possible accuracy. 

Fourth. The geodetic parts of this phototachymeter have 
been designed with a view to give results (vertical and 
horizontal angles) as accurate as those obtainable with 
GouHer's theodolite of lo cm. diameter, which instrument 
had been previously used for the triangulation of the 
Mont Blanc group and which was how to be replaced 
by this new combination instrument. 

Fiftli. The camera, with fixed focal length, commanding 
all angle of 40 centigrades above and below the horizon, 
was to be made of metal and the correct position of the 
horizon hne should be readily obtainable for every photo- 
graphic perspective. 

Sixth. The component parts of the phototachymeter have 
been assembled by the makers in such a manner that all 
future adjustments may be dispensed with. Any dis- 
crepancy may be discovered, however, by special obser- 
vations, made for testing the adjustments, which discrep- 
ancies may be neutralized by applying proper corrections 
to the results. 

Seventh. The lens selected should be as speedy as possible 
to reduce the time of exposure, and the definition of all 
points falling within the unfavorable parts of the plates 
should be correct within o.i mm. (The diameter of 
the circle of diffusion <o.i mm.) 

Eighth. If (orthochromatic) plates give better results than 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 255 

films the camera chamber should be arranged for the 
use of the former. The definition should be good enough 
to admit a twofold (photographic) enlargement with 
no distortion to affect the accuracy of the iconometric 
plotting. 
Ninth. iMeans were to be provided to enable the operator 
to inspect the field under control for each exposure with- 
out having recourse to dark cloth and groimd glass. 
Tenth. Provision should be made to divide the panorama 
automatically into sections of equal horizontal extensions. 
Eleventh. The exchange of plates, when photographing a 
panorama view, should be made without the least chance 
of disturbing the adjustments of the instrument. 
At first Carbutt's orthochromatic films were used, and it 
was foimd that they imderwent a change retraite during the 
developing process, which, however, was uniform in character, 
thus admitting a correction to be applied. Still, it appeared to be 
desirable to avoid even such a uniform change in the dimensions 
of the negatives, requiring a correction, and glass plates are now 
used altogether in Vallot's work. 

The orthochromatic plates are 130X180 mm. in size, with an 
effective field for the perspective of 120X170 mm. These small 
plates do not materially increase the weight of the outfit; one 
cardboard box containing 12 plates weighs 1.4 kgr. 

The double-plate holders (0.25 kgr. in weight) were made by 
Balbreck. They may be inserted into the carrier and the slides 
be withdrawn without imparting the least shock to the instrument. 
The 18 double-plate holders, carried in a special packing-case, 
are numbered from i to 36. The plates are marked in one comer 
with a soft pencil before removal, giving series and number. For 
instance: 4, i to 36; 5, i to 36, etc. 

While in the field, the exposed and imexposed plates (their 
plate-holders) are kept separated in the case by placing the ground- 
glass frame between them. The plates used at present are 
Lumiere's orthochromatic, series^ (sensitive to yellow and green). 



256 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The diagram given in Fig. 157, Plate LXXXII, represents a 
vertical section through the camera chamber (in the principal 
plane). 

^B=nodal plane of the lens; 
AC = CB'=^o mm. = range of possible vertical change of 
lens above or below its normal position (at C) ; 
ikfA7'= effective vertical width of plate = 120 ijim.; 
A = extreme upper position of lens; 
5= " lower " " " 
C= normal or central position of lens; 
A'MS'N=i cm.; 
44' = CC'= 55' = constant focal length = 150 mm. 

The extreme vertical angle which a plate may contain will be 
found from 

,,,^, A'N no 

tan A'AN=-r-r7 = =0.733, 

AA' 150 '"'"^' 

i^A'AN=^o.2)^ (centesimal graduation; 400*^ = 360° sexagesimal 
graduation). 

For the extreme lower position B the greatest vertical angle would 
be 

i.B'BM=Ao.f, 

and for the central or normal position C the greatest angle above 
and below the horizon is found from 

^,^,^ CM 60 

tan C'CM = -pw}=^ = =0.4, 

L'L 150 

^C'CJf=^C'CiV= 26.2^. 

The horizontal field a of the plate may be found from 

« 8s 
tan-= — =0.567, 
2 150 •' ' 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 2$^ 

the effective width of plate being = 1 70 mm. 



- = 32.8; a=6s.6<^. 



If no overlaps are required a panorama may be photographed 
on six plates, 

6x6s.6'^=393.6^, 

showing the omission of 6.4'' of the horizon. It often will be 
desirable, however, not only to photograph the entire panorama, 
but also to overlap two adjoining plates. The horizon has 
accordingly been divided into six equal sectors of 60'' each and a 
seventh sector of 4.0'^. 

In place of the customary sHde (which is either too tight or 
too loose and consequently not light-tight) the camera chamber 
has been provided with three openings (Oi, O3, and O2, Fig. 155, 
Plate LXXX), A, C, and B, Fig. 157, Plate LXXXII, into either of 
which the lens may be inserted and securely held there by a 
bayonet catch. The lens is secured in the middle opening O3, 
Fig. 155, when the vertical angles above and below the horizon 
do not surpass 26^. It is inserted at O2, Fig. 155, Plate LXXX 
(at B, Fig. 157, Plate LXXXII), when low grounds (valleys) are 
to be photographed from high elevations and at A, Fig. 157, 
Plate LXXXII, when mountain peaks are to be photographed 
from lower stations. 

The horizon line for each of these three positions of the lens is 
located on the negatives by a set of two projecting points p, Fig. 155, 
Plate LXXX, which are photographed on every plate. 

When great differences in altitude are to be recorded, two 
plates may be exposed in the same direction and from the same 
station, one with the lens at A, the other with the lens at B, Fig. 
157, Plate LXXXIL 

The openings Oi and O2, Fig. 155, Plate LXXX, are closed by 
caps which have the same bayonet catch as the lens mount O3. 



258 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The objective is an anastigmat of Zeiss, No. 2, series Ilia, 

with revolving diaphragm and stop No. 5 ( — ) . This lens is made 

of Jena glassby Krauss in France, and it has a constant focal 
length of 150 mm. 

Enlargements of the negatives 50X60 mm. give very good 
results for the iconometric plotting, which has been done on 
scale 1 : 20000 for the Mont Blanc survey. 

Behind the lens a yellow (plate) glass screen has been in- 
serted with a tint sufficiently dark to necessitate the length of 
exposure required for the normal plate to be increased fifteen times. 
The insertion of this screen increases the focal length of the ob- 
jective to 151 mm., the yellow light-rays being less refracted than 
the blue and violet rays. This camera is not provided with an 
instantaneous shutter, as the exposure will always require several 
seconds. 

The tripod is made of oak and it may be folded together to 
0.75 m. length. On rocky peaks, where the unfolded tripod 
would be too long or where it would require too much ground 
space, it may be used in its folded condition. 

The tripod head H, Figs. 155 and 156, Plates LXXX and 
LXXXI, is made of aluminum, which metal enters greatly into 
the composition of this instrument. The camera chamber and 
lens-hole caps Oi and O2, Fig. 155, Plate LXXX, are also made 
of aluminum. 

The horizontal circle and metal instrument support resembles 
in form the horizontal circle adopted for the tachymeter- theodo- 
lites used by the Genie Corps of France. The three leveling- screws 
L, Figs. 155 and 156, Plates LXXX and LXXXI, set into the 
three arms A, are on the circumference of a circle of 0.17 m. in 
diameter. 

The horizontal circle R, Figs. 155 and 156, Plates LXXX and 
LXXXI, is o.io m. in diameter and is mounted above the 
circular disc F which supports the cross-levels h and I2, used for 
leveling the instrument. 



SIIRVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 259 

Its graduation (centesimal 400*^ = 360° sexagesimal) is on the 
vertical (outer) cylindrical surface, and it may easily be read 
when the camera is mounted on the instrument support. 

"A cohical pinion (in the prolongation of the vertical axis) on 
the horizontal circle serves to support the camera or the tachym- 
eter-theodolite, which are provided with^onical bearings to fit 
this conical pinion. 

A special arrangement with stops insures the uniform azi- 
muthal swings of the camera when exposing the plates forming 
a complete panorama; six of these plates cover an angle of 60^ 
each and the seventh controls 40*^ of the horizon. Each plate 
covers or overlaps the adjoining one by a common margin of 
15 mm. in width. When used as a theodolite the arresting stops 
may be set inactive. 

The definition of the photographic perspectives for all objects 
from 10 m. to infinite distance is very clear (using stop No. 5). 

A tele-objective (long-distance objective) has been provided 
by means of which circular pictures may be obtained on the plate 
of o.ii m. diameter and with a two and one half fold enlargement. 

The metal points p and p^, Fig. 155, Plate LXXX, have cir- 
cular openings of i mm. diameter which serve to locate the 
horizon and the principal line on such pictures where the image 
of the plate has been obscured by the tint of the image close to 
the point. 

Blackened diaphragms D, Fig. 155, Plate LXXX, are inserted 
into the darkened chamber to intercept all side-light rays that 
may be reflected into the camera and cause the plate to be 
fogged. 

The heads of the three screws S, Fig. 155, Plate LXXX, 
used to secure the camera in its position on the horizontal circle, 
are corrugated or checked and well blackened to prevent side 
reflections of light-rays from their surfaces. 

The ground-glass plate is used only when testing the adjust- 
ments of this instrument. 

The upper face of the camera is provided with three sight- 



26o PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

frames Vi, V2, and V3, which may be revolved about their 
lower ends to lay perfectly flat on the camera top when not in 
use. 

Two vertical planes containing Vi, V2, and F1F3 include a 
horizontal angle of 60'', the extreme width of the horizontal 
field for one plate. "Jhe sights-frame Vi (near the lens) is, pro- 
vided with a vertical wire and the sight-frames V2 and V3 (at 
the back of the camera, Fig. 155, Plate LXXX) have three 
peep-sights each, which, together with the metal bridges of Vi, 
are disposed at such distances to correspond with the vertical 
angles, commanded by the lens on the plate, for the three differ- 
ent positions that may be given the lens. By sighting through 
the three peep-sights it may readily be determined into which 
one of the three openings (Oi, O3, O2, Fig. 155, Plate LXXX) the 
lens should be inserted to control the extension of a certain pan- 
orama section in the vertical sense. 

For executing the tertiary triangulation and for locating 
detached camera stations (stations not already included in the 
triangulation scheme) the surveying-camera, Fig. 155, Plate 
LXXX, may be converted into a tachymeter-theodolite by replac- 
ing the camera by an "alidade holom^trique " with "broken" 
telescope, abf Fig. 156, Plate LXXXI, hke that of Col. Gou- 
lier's pattern (with the exception that the ruler has been dis- 
carded here). The base of this tachymeter has three screws S, 
Fig. 156, Plate LXXXI, corresponding with those of the camera, 
to secure it to the horizontal circle. 

The telescope, which magnifies about twelve times, is "broken" 
to faciUtate the measuring of large vertical angles. A finder e, 
Fig. 156, Plate LXXXI, of a fourfold magnifying power is pro- 
vided. It has the same form as the telescope ab. 

Vertical angles are measured with the vertical circle E 
(" I'&limfetre"), Fig. 156, Plate LXXXI, which has a diameter 
of 0.08 m. and reads to single grades (quadricentesimal gradu- 
ation). 

A special level is supplied to facilitate the ready adjustment 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 261 

of the telescope ab into horizontal plane when this tachymeter 
is to be used as a spirit-level. 

This entire phototachymeter (excluding the tripod) is packed 
into a box having three compartments, which may be closed by 
two doors like a wardrobe. One compartment contains the 
tachymeter proper (" I'&limetre ") ET, Fig. 156, Plate LXXXI, 
the other receives the camera C, Fig. 155, Plate LXXX, and 
the third is reserved for the repeating circle R with its support A, 
Figs. 155 and 156, Plates LXXX and LXXXI. 

The folding tripod, together with a hght folding stadia or 
telemeter-rod, is carried in a separate packing-case. All pack- 
ing-cases are covered with rubber cloth to guard against the 
evil effects of dampness. 

This entire instrument outfit may be strapped to a light 
packing-frame, making a compact pack to be carried on the back. 

The weight of this outfit has been distributed as follows: 

Complete phototach\Tneter 5.3 kgr. 

Packing-case and accessories 3.6 

Water-proof case, including straps 0.8 

Folding tripod 4.8 

Folding stadia-ix)d 0.8 

Water-proof case for both, including straps ... 0.7 

Total weight of instruments 16 kgr. 

Packing-frame, including straps 2 " 

Total weight to be borne by one packer. . . 18 kgr. 

The 18 double-plate holders (including 3 doz. plates) weigh 
9 kgr.; they are stored in a wooden case which weighs 3.5 kgr., 
including the water-proof covering. These 12.5 kgr. are carried 
by a second packer who has additional implements to take, 
bringing the weight of his pack also up to 18 kgr. 

For the survey of Mont Blanc it has been found that 36 
plates fuUy suffice for a day's work. For a trip of several days' 



262 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

duration the observer carries extra plates (in their cardboard 
boxes, but stowed away in a separate water-proof packing-case), 
which are exchanged every night, using a small folding ruby 
light and portable dark tent. 

In 1894 four hundred' negatives were obtained by Vallot 
Bros., and in 1895 the number reached over 500. The season 
of 1896 was so misty and rainy that few days were available 
for this work, the mountain peaks being rarely visible, and also 
lower parts of the mountains being more generally hidden from 
view in a dense fog. This season's results again proved the value 
of the phototopographic method above all others for surveys 
in the higher altitudes of mountainous regions, as the results 
obtained during the short periods of good weather could not ■ 
have been acquired in the short time available by any other 
known topographic method. 

The area of this survey is controlled by 300 triangulation 
points (established between 1894 and 1896) which are connected 
with a base line at Chamounix of 1785.824 m. length. 

P Phototheodolite Designed by J. Bridges-Lee. 

This instrument has been patented in England and other 
countries, and it is made by Louis P. Casella, who has published 
a full description of the same, with instructions for its use in 
the field (Description of a New Phototheodolite designed by J. 
Bridges-Lee, Esq.,.M.A., F.G.S., etc. With full instructions 
as to its manipulation in the field. L. Casella, maker to the 
Admiralty, Ordnance, etc.. No. 147 Holborn Bars, London, 
E. C), from which the following description has been abstracted: 

This phototheodolite is shown in Figs. 158 and 159, Plates 
LXXXIII and LXXXIV, the latter representing a view of the 
interior of the camera-box, the ground-glass plate H having 
been turned down. It will be noted that this instrument com- 
prises : 

(i) A complete, well-made theodolite reading to minutes; 



SURVEYING-CAMERAS AND GEODET C INSTRUMENTS. 263 

(2) A photographic outfit answering all ordinary demands; 

(3) A good and large azimuth compass, its vertical scale 

admitting of very close readings to be made either 

through the rear windovif h' or through the lens B. 

The combination of these three instruments into a symmetrical 

well-made phototheodolite makes this one of the most generally 

■useful instruments that an explorer of unknown regions can 

add to his instrumental outfit. 

With reference to Figs. 158 and 159, Plates LXXXIII and 
LXXXW, we have: 

A. Rectangular camera-box made of aluminum (cast metal); 
its upper face supports the telescope E, with vertical circle F 
reading to minutes. This box is permanently attached to 
the leveling support T, Fig. 158, Plate LXXXIII. 

B. Rectilinear photographic lens of excellent quality and sup- 
plied with iris diaphragm. No focusing adjustments are 
pro\ided for this lens, and it should be set by the instru- 
ment-maker very accurately in correct position with refer- 
ence to the other parts of the instrument. 

A second photographic lens, movable in a sleeve with rack- 
and-pinion focusing adjustment, will be suppUed if the instru- 
ment is to be used for ordinary photographic work at short dis- 
tances. 

C. Horizontal circle, graduated to half degrees with vernier 
reading to single minutes. 

The vernier is attached to the rear of the camera, Fig. 159, 
Plate LXXXIV, its zero mark falhng into the same vertical 
plane which passes through the optical axis of the camera-lens. 

D. Tribrach, or triangular piece connecting tripod head and 
camera. It supports the terminating heads at the base 
ends of the three leveling- screws. To insure stabiUty the 
terminal feet of the levelling-screws may be secured to the 
tribrach by means of the usual locking-plate connected 
with the tribrach. 

E. Telescope with cross- and stadia-inches in the body and 



264 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

general adjustments for a surveying-telescope. The tele- 
scope is mounted like that of a plane-table alidade to be 
rotated about a horizontal axis in a vertical plane only. Its 
plane of transit contains the optical axis of the camera-lens, 
the zero mark of the vernier of the horizontal circle, the ver- 
tical hair K, as well as the vertical axis of the needle sup- 
port of the azimuth compass, when the instrument is in 
adjust ment and leveled. 

F. Vertical circle, graduated into half-degrees, firmly con- 
nected with the telescope and provided with a fixed vernier 
reading to single minutes. This circle controls a vertical 
range sufficient for all ordinary topographic surveying pur- 
poses. 

G. Tubular spirit-level revolvable in a low cylindrical case 
firmly attached to and sunken into the upper face of the 
aluminum camera-box. This level serves to give the top. 
of the camera-box a horizontal position when the camera 
has been oriented for an exposure to be made. This adjust- 
ment is made in the usual manner by means of three leveling- 
screws supporting the instrument on the tribrach. . 

H. Back of the camera containing the ground glass h and sup- 
pHed with hinges to enable the operator to swing the ground 
glass back, as shown in Fig. 159, Plate LXXXIV. A win- 
dow h' of poUshed glass is inserted into the ground glass h 
to enable the surveyor to observe the vertical index hair K 
and coincident graduation mark of the vertical compass 
scale, with or without the use of the microscope O. 
I. Rectangular metal frame supplied with stout backstays 
(not visible in the illustrations) and securely attached to the 
bottom plate which supports the compass box M. This 
bottom plate may be moved in the direction of the axis 
of the camera-lens, sliding on guides rigidly fixed to the 
bottom of the camera-box! 

When the camera is to be used for ordinary photographic 
purposes the bottom plate (including the frame I and azi- 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 265 

muth compass) may be withdrawn and laid aside. A square 
piece of black velvet is provided to be placed upon the metal 
bottom of the camera-box. After the insertion of the second 
lens with adjustable focal length the camera will be ready 
to be used for photographing near objects. 

The rear surfaces of the frame I are in vertical plane 
which is perpendicular to the optical axis of the camera- 
lens when the instrument is leveled and in perfect adjust- 
ment. 
jj. Transverse pinion extending across the bottom of the camera- 
box and terminating in two milled heads J J. The frame / 
may be racked backwards or forwards by a corresponding 
turning of the milled heads //. 

Pointers are attached to the milled heads, which serve 
to indicate whether the internal structures are in the forward 
or backward position when the falling back of the camera- 
box has been let down to allow a double-plate holder to be 
inserted into the camera. 
The dimensions of the rectangular frame / are such that 
it can he. racked back (entering the plate-holder frame) to be 
brought into direct contact with the sensitized film of the photo- 
graphic dry-plate, when the cross-hairs K and K' will also actually 
touch the fihn. Of course, the slide protecting the photographic 
plate against light should be first removed before the frame / 
is racked back. Two small stops in the form of shding bolts 
(not shown in the illustrations) are provided to prevent the frame 1 
from being carried back too far or with too much force and 
to secure uniformity of the focal length for all phototopographic- 
plate exposures. 

K. Vertical hair passing through two round holes in the frame 
I and held in position by means of small wooden pegs. 
When broken, a new hair can readily be inserted into the 
holes and secured by small pegs. This hair serves to mark 
on the negative the median vertical plane of the Lastrument 
and the principal line of the photographic perspective. It 



266 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

cuts the optical axis of the camera-lens at right angles and 
it serves as index mark when the compass scale is read. 
It is in the same plane as the vertical web in the telescope, 
the optical axis of camera-lens, the vertical axis of revolu- 
tion of the azimuth compass, and the zero mark of the vernier 
of the horizontal circle. 

K'. Horizontal hair secured to the frame in the same manner 
as hair K. The point of intersection of both hairs is in 
the optical axis of the camera-lens, and on the negative it 
marks the principal point of the photographic perspective. 
When the camera is in perfect adjustment and accurately 
leveled the shadowgraph of K' on the negative will bisect 
points having the same height as the optical axis of the 
camera at the station whence the picture was taken. 

The proper location of the small holes in the frame I, 
fixing the position of the hairs K and K', is ascertained by 
the maker. 
LL. Small tablets of transparent celluloid or xylonite. They 
serve to receive notes giving particulars concerning station 
mark, barometric reading, or determined elevation of camera 
station, number of plate, time of exposure, etc., which it 
may be desirable to record photographically, as shadow- 
graphs, on the picture. Such notes are written upon the 
tablets LL in the ordinary way with quick-drying ink. 
The tablets when dry are placed upside down in small pock- 
ets in the frame and the notes appear as shadowgraphs in 
the corners bordering the sky portion of the negative. 

M. Magnetic azimuth compass with vertical cylindrical trans- 
parent scale, graduated to half degrees from o° to 360°. 
This transparent scale in its revolution passes quite close 
to the vertical index hair K, without ever touching it, how- 
ever. The pivot of the compass is permanently secured 
to the base or bottom plate already referred to, so that the 
cylindrical compass-scale is always at exactly the same 
distance from the vertical hair K. 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 267 

When the bottom plate supporting the frame / and ■ com- 
pass M is racked forward in the camera-box, a copper disc 
is automatically raised, lifting the agate cup and pressing it against 
the support m so that the compass is firmly clamped, preventing 
injury to the pivot and other parts of the compass during trans- 
portation. When the bottom plate is racked back to bring the 
cross-hairs K and K' into contact with the sensitive film of a 
photographic plate, the agate cup is lowered (automatically) 
upon the pivot and the magnetic compass assumes its natural 
position. The compass graduation being on a transparent 
cylinder, the magnetic azimuth of the principal line of every 
exposed plate will be recorded photographically in the sky region 
of the negative as a shadowgraph. 

N. Catch to hold the double-plate holder in place when inserted 
into the camera. The frame of aluminum forming the 
rear of the camera-box is faced with black velvet to guard 
against the entry of extraneous light when the double- 
plate holder is in position. r 
O. Microscope with imiversal joint movement to permit of 
its being used either for reading the observed horizontal 
angles (on the horizontal circle C with vernier), or for read- 
ing the compass bearings through the window h' in the 
groimd-glass back h. 
P. Adjustable microscope for reading vertical angles. 
Q. Clamp- and tangent-screw for arresting azimuthal or hori- 
zontal circle and giving it slow motion. 
R. Clamp- and tangent-screw for camera. 
S. Clamp- and tangent-screw for telescope. 
T. Strong aluminum head of tripod with bronze clamping- 
screws for folding legs. It is suppHed with transverse bars 
(not shown in the illustrations) of bronze, serving as attach- 
ments for chains to safeguard the instrument when in posi- 
tion at a station. These bars also serve to receive the hooks 
attached to a net into which heavy stones may be placed 



268 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

to give stability and steadiness to the instrument when used 
in windy weather. 
U. Two cross-marks on the top of the box. Their distance 
indicates for ordinary temperatures the permanent focal 
length of the camera, to be used for the negatives. 

A small hook for the attachment of a small plummet is secured 
to the tripod head T in the central axial line of the instrument. 
The telescope is supplied with an erecting as well as an inverting 
eyepiece. 

A color-screen of optically worked green glass may be fitted 
inside the sunshade of the photographic lens. Yellow or orange 
glass can also be supplied when desired. 

Attached to the frame I (which carries the hairs) is a hori- 
zontal transparent scale of angular distances (Fig. 159, Plate 
LXXXIV), photographically prepared by aid of the identical 
lens and instrument which it accompanies. It is used for sur- 
veying purposes, as with its aid the exact angular distances of 
points in the picture — to the right or left of the principal vertical 
plane — may be read off directly with the aid of parallel rulers. 
This scale also facilitates the determination of compass errors, 
because if there are any points in a picture whose true bearings 
have been fixed with precision — trigonometrically or otherwise — 
it is only necessary to add or subtract the angular distances of 
those points, as read on the horizontal scale of angles, to or 
from the photographiccally recorded compass-bearings of the 
points, and the difference between the compass-bearings and 
the true bearings is the compass variation. This simple verifi- 
cation can be performed in office at any time. This instru- 
ment is suppUed with 6 double-plate holders of good construction, 
to carry one dozen plates, size 5X4 in., either horizontally or 
vertically. 

It fits easily and securely in a strong, well-made, brass-bound 
mahogany case with catches, lock, and key. 

The double-plate holders, extra eyepiece, €xtra camera-lens, 
color-screen, and plumb-bob all fit in the same case, which for 



SURVEYING-CAMERAS AND GEODETIC INSTRUMENTS. 269 

greater security and convenience of transportation is placed into 
an outer sole-leather case with pack-straps. 

The tripod head is provided with a metal screw-cap, a suit- 
able protecting cover of its own, and the legs can be strapped 
together for easy transportation. 



CHAPTER IX. 

PANORAMA CAMERAS. 

The older lens types gave correct perspectives only for small 
angles, rarely exceeding 30 degrees, and Martens in Paris was 
probably the first to devise a so-called panorama camera capable 
of photographing wider sections of the horizon on one plate, 
with lenses that ordinarily would cover but a small angular field. 
He solved this problem by constructing a hemi-cylindrical camera 
with a revolvable lens plate. If the objects are far enough away 
to permit the use of a constant focal length of lens, and if the 
lens may be rotated about a vertical axis in the second nodal 
ooint of the lens system, panorama views may be obtained on a 
sensitized surface of a daguerreotype plate bent into a half-cylinder 
whose radius equals the constant focal length of the lens and 
whose axis coincides with the vertical passing through the second 
nodal point of the camera-lens. 

I. The Photographic Plane Table Devised by A. Chevallier (1858). 

CheyaUier's " planchette photographique " may be men- 
tioned here, as, in a certain sense, it also is a panoramic camera. 
In this instrument the entire panorama view was continuous 
and found representation on a single plate; the latter, however, 
was exposed in the horizontal plane. The lens axis being hori- 
zontal, a prism had to be interposed between plate and lens to 
bring the image into the horizontal picture plane. All verticals 
of the landscape converged to one point, the center of the cir- 
cular horizon line. For further deails of this historically inter- 

270 



THE ROCKWOOD-SHALLENBERGER PANORAMIC CAMERA. 27 1 

esting instrument we would refer to the publications on this sub- 
ject given under French phototopographic literature. 

II. The Rockwood-Shallenberger Panoramic Camera. 

A horizontal section through this camera (made just above 
the camera-lens) is represented in Fig. i6o, Plate LXXXII. It 
practically consists of two cameras C and c. The latter (smaller) 
one contaias the lens O and is revolvable about a vertical axis 
passing through the latter. The main camera-box C forms a 
semi- cylinder with a sensitive film stretched over the inner cylin- 
drical surface, and that may be unwound from the magazine 
roller B passing to the receiving roller A after" exposure. As 
the small camera is revolved, the light-rays entering the lens 
act upon a narrow vertical strip of the film at a time. The con- 
nection between the objective end of the small camera c with 
the front board b of the main camera C is accomplished by means 
of a pHable light-tight fabric e. The lens O has a long focus 
and the panoramic perspective is entirely free from distortion, 
only a vertical strip of one quarter inch width being exposed at 
one time. The pictures are 8X40 inches and it takes from 
three to five seconds for the lens to complete one revolution of 
180 degrees. The speed in the swing of the smaller camera 
is controlled by a clockwork, the rate of which may be increased 
or retarded at pleasure, with due reference to the changes in 
atmospheric conditions and character of subject. 

III. R. Moessard's Topographic Cylindrograph. 

The so-called cyHndrograph of R. Moessard (commandant 
du Genie, attache au Service geographique de I'armee) is 
similar in construction to the apparatus just described; . this 
instrument, however, is specially devised for surveying purposes. 

The semi-cylindrical camera- box, Fig. 161, Plate LXXX\', 
rests upon a tripod with leveling-screws to adjust the verticalit\' 
of the axis of revolution aa of the camera lens O, which axis coin- 



272 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

cides with the axis of the half-cylinder, formed by the surface 
of the sensitive film. For focusing purposes the latter may 
be replaced by a semi-cylindrical ground-glass plate. By using 
the sight-ruler 5 as a lever the camera-lens O may be rotated 
about aa, allowing the speed of motion to be controlled by the 
illumination of the landscape. By carefully examining the 
panorama through PP' while aa is being moved in azimuth, 
the correct timing for the exposures of the different panorama 
strips may be made. The space between the frame RR is filled 
in with a soft and light-tight fabric, allowing an easy play for 
the rotation of the objective 0. 

The upper surface of the topographic cylindrograph is pro- 
vided with an azimuth compass C and a set of cross-levels A 
and B. The bent frame forming the guide for the film is sup- 
plied with graduations on the inner edges which form the mar- 
gins of the panoramas. The divisions of the upper and the 
lower scales (horizontal) correspond to degrees in arc, while 
the divisions of the vertical marginal ends are graduated to read: 

— , where / = constant focal length of the lens O ( = radius of 

100 ° ^ 

the cylindrical sensitive surface of the film). Four movable 
indices are provided; two of these, H and H', Fig. 162, Plate 
LXXXVI, serve to mark the horizon line of the half-panorama 
and the other two, N and E, serve to indicate^ the magnetic 
meridian and the magnetic east and west line, respectively, 
for each panorama view. The proper placing of the indices 
for each half-panorama may be accomplished by means of the 
azimuth compass C and sight-ruler or alidade S. Thus the mag- 
netic azimuths of horizontal directions may be taken directly 
from the picture. 

The vertical angles are readily found by means of the ordinates 
of the pictured points (above or below the horizon line HH') 
measured in one hundredths of the focal length /, using the photo- 
graphed scales on the vertical margins of the pictures for this 
purpose. 



R. moessard's topographic cylindrograph. 273 

For instance, the angle of depression of the ray Oa (to the 
foot of the pictured tree a, Fig. 162, Plate LXXXVI) is found 
from 

tan/3'=-T-, 

or when aa' is measured on the side scale and found to be 25 parts, 

tan B = =0.2=;. 

^ 100 -^ 

To determine whether the levels A and B, Figs. 161 and 163, 
Plates LXXXV and LXXXVI, read zero when the cylindrical 
film is vertical, and also to ascertain whether the indices H and H', 
Fig. 162, Plate LXXXVI, representing the horizon line are cor- 
rectly placed, we may proceed as follows: 

A theodolite. Fig. 163, Plate LXXXVI, is set up about 10 
or 15 metres behind the cylindrograph (after the back of the 
camera had been removed to bring the indices H and H' into 
view) and both instruments are leveled. After bisecting the 
upper edge of the cylindrograph and the telescope of the theo- 
dolite is moved in azimuth the bisection should continue, and 
the same should be the case for the lower surface edge of the 
cylindrograph. If this does not take place, then the cylindrograph 
should be adjusted by means of the leveling-screws until the 
bisection does take place and the level A is then changed to 
read zero. The theodolite is now set up in the direction .ob the 
level A (at one side of the cylindrograph) and the leX'Sr'B is- 
adjusted in a similar manner as just described for A. 

To adjust the indices H and H' into the horizontal plane- 
containing the optical axis of the adjusted cyUndrograph a com- 
parison may be made on a cylindrograph picture showing several 
points of known elevations, the elevation of the cyKndrograph 
being also known, or the theodolite may be set up with the hori- 
zontal telescope at the same elevation as the optical axis of the 
adjusted cylindrograph. The horizontal telescope of the theo- 
dolite is now moved in azimuth until a well-defined point is 



2 74 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

bisected, which point may be identified on the ground glass of 
the cylindrograph. The image of this point on the ground 
glass is marked and the cyhndrograph is moved in azimuth, 
marking the image on the ground glass in two more places. A 
(horizontal) Une passing through these marked points should 
pass through H and H'. 

The objective is attached to a funnel-shaped box situated 
within the camera (see Fig. i6o, Plate LXXXII) and permitting 
the simultaneous exposure of a vertical strip of film of a width 
of but 62 mm. Points of the film that would be pictured out- 
side of this strip cannot be acted upon by the light-rays until 
the objective be revolved (about the axis aa) sufficiently far 
to expose them to the effects of the light. After the time needed 
for the correct exposure of this strip (of 62 mm. width) has 
been ascertained (by experiment or otherwise) the correct expos- 
ure may be given the entire semi-cylinder by moving the sight- 
ruler S, with a quick uniform motion, about aa, from one extreme 
end of the film to the other. The semi-cylindrical film being 
860 mm. long, each strip of the film would then have been exposed 
the ^2/ggQth part of the time required to make one full revo- 
lution of the objective. If one complete revolution required 
10 seconds, and if the correct exposure for the strip was found 
to be 5 seconds, each strip would have received an exposure of 

-■^ seconds=o.72 second. To give each strip the required 

exp(K.4^< of 5 seconds the entire revolution of the lens should 

be repeated times, or about seven times, each revolution 

^ 0.72 

taking 10 seconds. 

These panorama instruments are not made sufficiently pre- 
cise, in their present form at least, to be recommended for topo- 
graphic Surveys. Moessard's cylindrograph, however, is well 
conceived, and where the transportation is an easy one, the 
topographic cylindrograph, in a more perfected form, may give 
results sufficiently accurate for surveying purposes. 



CHAPTER X. 
ICONOMETERS AND PERSPECTOGRAPHS. 

Under iconometers we comprise a series of instruments 
which have been devised to simplify the constructions of photo- 
topographic plotting or iconometry. 

After two drawing-boards have been covered ' with paper 
(gummed down on the edges), both sheets are provided with a 
chart projection upon which all trigonometric (triangulation) 
points are plotted and their elevations inscribed. 

The construction incidental to the iconometric plotting of 
the phototopographic survey may be divided into three classes: 
First. The plotting of all horizontal directions that had 
been observed, instrumentally, for the location of the 
camera stations and for the orientation of the panorama 
views. 
Second. The determination of the horizontal projection of 
points pictured on three or more photographs, taken 
■from different stations. 
Third. The determination of the elevations of the various 
camera stations and tertiary points that are located icono- 
metrically to facilitate the plotting of the horizontal con- 
tours of the terrene. 

I. Graphic Protractor (of L. P. Paganini). 

With the aid of a specially constructed protractor, Fig. 164, 
Plate LXXXVII, and tracing-paper the directions obtained 
with the theodolite or transit in the field can readily be plotted 

27s 



276 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

upon both the working- and chart-sheets. This protractor, 
represented in Fig. 164, Plate LXXXVII, consists of two con- 
centric rings AA and BB, the former being movable within the 
latter about the common axis C, secured in the center of AA 
by means of the plate aa. The rotary motion is applied to A A 
by means of two projecting ribs as and sa on the plate aa. 

The inner circle A A has a graduation divided into degrees 
and half degrees, while the outer circle BB bears a vernier w, 
reading to half minutes, the zero of which lies in the prolongation 
of the fiducial edge of an arm hh, the latter being permanently 
secured to the outer circle BB and in a radial position to the 
same. The clamp-screw P serves to hold the two circles in 
any position. 

An alidade ruler, DD, the fiducial edge of which also passes 
through the center C, common to both circles, is revolvable about 
the axis C and it may be moved over the upper surfaces of the 
two circles AA and BB. This ruler, DD, bears a vernier n', 
graduated like n to read to half -minutes and its zero coinciding 
with the fiducial edge of DD. The clamp-screw P' serves to 
clamp this movable arm DD to the outer circle BB. 

The axis C has a conically shaped hollow interior, at the 
bottom of which a thin piece of isinglass or horn is secured in 
such a manner that it may be removed for renewal whenever 
the small puncture indicating the center of the circles and axis 
of revolution be worn too large. 

When using an ordinary protractor to lay off the various 
directions (radials, that were observed with the transit in the field) 
from one camera station, much valuable time will be absorbed 
in making the additions and subtractions (which have to be 
made in order to obtain the actual values for the successive angles 
between such lines of direction), particularly when a series of 
panorama stations are to be plotted. 

The protractor, as shown in Fig. 164, Plate LXXXVII, may 
be used not only as an ordinary protractor — by bringing the 
zeros of both circles to coincide and clamping the two circles 



L. P. PAGANINl'S GRAPHIC PROTRACTOR. 277 

in that position, by means of the clamp-screw P — but it may 
also be used to plot the directions upon the map or working- 
sheet in the same manner as they were obtained in the field with 
the transit; that is to say, they may be referred to zero or to 
any other direction as the beginning, and then be plotted in 
successive order. 

To do this, the inner circle is revolved until the zero of BB 
(vernier n) gives the same reading upon the graduation of the 
movable circle ^^ as the recorded reading on the transit for the 
prime direction. Now both circles are clamped together by 
means of the clamp-screw P. The line of prime direction is 
"drawn along the fiducial edge of the fixed ruler hb upon the work- 
ing-sheet (or upon tracing-paper if the station is to be located 
or fixed upon the tracing of the lines), the center C of the instru- 
ment coinciding with the point representing the station upon 
the paper. 

The zero of the vernier n' of the alidade DD is then brought 
successively (upon the inner-circle graduation) to the readings 
of the other directions which radiate from the plotted station 
point at C, each direction being plotted in successive order by 
drawing a pencil line along the edge of the alidade DD. Care 
must be exercised not to change the primary position of the 
instrument as defined by the first line during all subsequent 
motions of the aUdade ruler DD. 

With the aid of this instrument the radials from the plotted 
camera stations may be obtained with rapidity and accuracy. 
If we have a sufficient number of directions to well-determined 
points which are evenly distributed about the station, their 
corresponding intersections upon both drawing-boards may be 
plotted with as much rapidity and accuracy as a graphical plotting 
will admit of. 

This protractor may also serve to locate points on the con- 
struction board that, on account of importance or for reasons 
of control, had been bisected from several stations with the 
transit, and also, as will be shown, to orient a perspective view 



278 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

(the picture trace) upon the board, if such perspective contains 
no triangulation point, or when the picture of such point is blurred 
or not sufficiently well defined to be identified with precision. 

After all stations, including such secondary and tertiary 
points that were determined by transit observations from the 
several camera stations, have been plotted upon the two boards, 
the work of iconometrically determining, upon the working- 
sheet, such points as seem needed to complete the map is taken 
up. For this purpose the various elements of the perspectives 
are tested and corrected, if needed, after the manner previously 
described, and all tertiary points are selected, identified, and 
marked, searching for well-defined points common to two or 
more plates, carefully selecting therefrom only such as appear 
to be the most useful, either for drawing the contours or for 
tracing the general trends of mountain ranges, torrents, and 
streams, boundary lines of glaciers, etc., the number to be 
selected depending greatly upon the character of the terrene, 
upon the adopted scale, and upon the accuracy to be attained. 
All tertiary points are marked upon the prints (perspectives) 
by numerals, letters, or symbols in red ink. 



II. L. P. Paganini's Graphic Sector (" Settore Grafico "). 

Instead of actually drawing the horizontal projections of all 
perspectives (the picture traces) upon the working-sheet, much 
time may be saved by using the instrument represented in Fig. 166, 
Plate LXXXVIII, devised by Paganini, who termed it "settore 
grafico," or graphic sector. With this graphic sector the hori- 
zontal directions to points marked upon the prints may be drawn 
cirectly on the horizontal plan without first drawing the picture 
traces. 

In Fig. 165, Plate LXXXVII, 

V represents the station, plotted on the working-sheet; 
OO' the horizontal projection of a perspective (the picture 



L. P. PAGANINI S GRAPHIC SECTOR. 



279 



trace, oriented with reference to the plotted triangulation 
point S) ; 
/= focal length for the perspective 00'; 
P= principal point (of view) of the perspective; 
Ps upon 00' is the measure of orientation of the perspective, 

corresponding to the azimuthal angle w; 
VP is perpendicular to 00' and =/. 
We now prolong VP beyond V by VP=VP'=j and erect a 
perpendicular to VP' = 0"'0" in P'. Produce, likewise, VB, 
VA, VS to their intersections with 0"'0", which intersections 
are marked b', a', and s', respectively. 

VP'^VP 

and 00' parallel to 0"'0"; 

hence the rectangular triangles VP'a', VP'V, and VP's' are 
congruent with VPa, VPb, and VPs respectively. Therefore 

P'a! = Pa = x, 
P'b'=pb=x' 
and P's'=Ps, 

giving also the measure of orientation ( = w) of the perspective 
of the picture to the picture trace. 

The screw e serves to clamp the screw m whenever the posi- 
tion of T with reference to V is to be fixed, after it has been 
brought to the desired distance from the center of rotation V. 
Two thumb-screws W and W (with hollow centers into which 
fine needles may be inserted to hold the sector in place after 
having been adjusted over a plotted station) serve to secure the 
metal sector in any desired position upon the drawing-board. 

The arc SS' of the sector is graduated to ten minutes, and 
the zero of this graduation coincides with the axis VP of the 
instrument, giving readings from 0° to 25° to either side of VP. 

The ruler or alidade RR' is provided with a vernier V, by 



28o PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

means of which the arc readings may be obtained within 30 
seconds. The thumb- or clamp-screw Z of the alidade has a 
counter plate at its lower end to secure the end B! of the alidade 
ruler upon the arc ss' of the sector and upon the steel ruler T. 

In order to draw the lines of direction upon the construction 
board to a point of the terrene (the picture of which had been 
identified and marked upon the perspective) the instrument is 
placed with its center of rotation, F, over the needle, marking 
the camera station on the working-sheet, and the button r is 
given a quarter-turn (care must be taken that the side bearings 
of the button r of the instrument may have no loose play about 
the needle), then T is moved by turning the screw m until 00' 
is distant from the center V by VP = j, whereupon the orientation 
of the instrument is accomplished as follows: 

RR' is to be directed to bisect a plotted triangulation point 
the image of which appears on the perspective sufficiently dis- 
tinct to serve as a reference point; its abscissa is taken from 
the photograph by means of a pair of dividers and plotted, in 
the inverse direction, upon the line 00' from the puncture, 
marking P; the alidade ruler RR' is now gently brought into 
contact with the other point of the dividers and it is secured in 
this position by clamping Z. 

Now the entire instrument is revolved about V until the 
end R of the aUdade bisects the plotted triangulation point, when 
VP will indicate the direction of the principal line and 00' will 
be parallel to the picture trace, which really would fall beyond V 
at a distance from V=VP=j. 

The instrument is secured in this position by gently turning 
the screws. The construction of the graphic sector, Fig. 166, 
Plate LXXXVIII, is based upon the preceding consideration, 
and it serves to draw from the station point V, on the horizontal 
plan, the various horizontal directions to points pictured on 
the panorama views. 

The metal plate VSS', shaped like a sector, may be revolved 
on the surface of the working-sheet, about the center of a. 



L. V. PAGANINl'S GRAPHIC SECTOR. 28 1 

needle, puncturing the plotted station in the center r of the 
sector. 

This needle passes through an oblong opening (of the same 
width as the diameter of the needle) of a revolvable button at r, 
secured in T', and through a similar slot at F in the metal sector 
plate VSS'. The metal ruler RR' is revolvable about V, gliding 
with the end R' over the arc SS' of the sector plate. The fiducial 
edge of the ruler RR' passes through the center of V or r, where 
it is secured to the revolvable button r by means of a cylinder, 
the bottom of which is provided with a slot similar to those in 
the button r and sector plate just mentioned. 

When the ruler RR' and the button r are in a certain position 
these three slots (in sector plate, button, and ring of ruler) will 
coincide, one falling above the other, and the needle may then 
be inserted through the three superimposed slots into the sta- 
tion point under V, the center of rotation. By a quarter-turn 
of the button r the needle will become inclosed in a square, of 
which the needle circumference will form the inscribed circle. 
The entire instrument may now be revolved about the needle 
center in V. 

The lever-screw m serves to move the steel ruler T parallel 
with itself and vertical to the axis nn' of the screw m. The axis 
of the screw m coincides with the direction of the central axis 
of the sector which passes through V and the middle of the arc 55'. 
When m is turned the ruler T glides up or down, its ends moving 
along the -grooves u and u', the inner edges of u and u' being 
graduated, so that the distance of the edge 00' of the ruler T 
from V may be read off to o.i mm. 

If the edge 00' of the steel ruler T is brought to a distance 
VP = } from the camera station in the center of V, by turning 
the screw m it will represent the trace of a picture with the focal 
length / (in inverse position, however, as it will correspond to 
the horizon line as viewed upon the ground-glass plate of the 
camera) (see Fig. 165, Plate LXXXVII). 

The point P, intersection of the axis of the instrument with 



282 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

00', will represent the principal point, plotted in horizontal 
plan. It is marked on the edge of 00' by a small conical cavity 
to receive the point of one arm of the dividers when the abscissae 
of pictured points are transferred from the horizon line. W and 
W are now pressed down, whereby the fine needles in the 
centers of W and W are pressed into the working-sheet. The 
end R' of the alidade is now released and the abscissae of all 
points, identified and marked on the perspective, are transferred 
to the line 00' from P, by means of the dividers, in their succes- 
sive order but in inverse direction (the fiducial edge of the 
alidade RR' being gently brought into contact with the point 
of the dividers each time), and the lines of direction are drawn 
along the fiducial edge of the ruler end R with a well-pointed 
hard pencil. 

Should the image of the triangulation point be indistinct 
or appear blurred upon the perspective, the instrument will 
have to be oriented upon the drawing-board by means of the 
angle of orientation ( = w) of the photograph, which angle had 
been observed in the field (in the Italian survey that angle is 
recorded in the field book, Model I, Chapter VIII). 

The end R' of the alidade is placed and secured in such a 
position that the fiducial edge of RR' forms the angle w with 
the axis VP of the instrument, which angle is laid off (in the 
inverse direction of the one observed) on the arc SS' of the sector 
by means of the vernier v. The instrument is then revolved 
about the needle in V the same as before, until the end R of 
the ruler passes through the trigonometric point in question 
marked on the plotting-sheet. The instrument having been 
secured in this position, by turning the screws W and W, is 
used in the same manner as just now described for drawing 
the radials which served to lodate the pictured points on the plan. 

If a plate had been exposed while the vertical thread (prin- 
cipal line) bisected a triangulation point, the angle w becomes 
zero and the orientation of such photograph trace on the plot- 
ting-sheet may be accomplished by bringing the zero of the 



L. P. PAGANINl'S GRAPHIC SECTOR. 283 

alidade vernier v to coincide with the zero of the arc gradua- 
tion, SS', clamping RR! in this position and directing the end R 
of the ruler to bisect the plotted triangulation point in question 
and securing the sector upon the working- board in this position. 

Should, finally, the perspective of which the trace in hori- 
zontal plan is to be plotted contain no images of points pre- 
viously located ahd plotted, then the zero of v is again made 
to coincide with the zero of the arc SS' and the instrument is 
revolved about the center of the needle V until the fiducial edge R 
of the alidade coincides with a line that had been drawn from 
the plotted station by means of the graphic protractor previously 
described, which forms in V (station point) an angle with the 
horizontal direction to some triangulation point observed in the 
field and equal to the angle of orientation ( = w) of the plate. 
This angle is taken from the field note-book (Model I, Chapter 
VI, I-C-6) and laid off on the sector in the inverse direction 
and the sector is again oriented in the manner shown before. 

After the horizontal directions to the different points, iden- 
tified on the panorama pictures, have been drawn with the graphic 
sector, they are provided with numerals or symbols to correspond 
with the designations affixed to the points upon the panoramic 
views, in order to facilitate their identification when seeking 
for the subsequent intersections with radials to the same points 
from other camera stations. The positions of the secondary 
and tertiary points on the plotting-plan are secured by inter- 
sections, as has been described in the preceding chapters, and 
they serve to make up the control of the map. It is well to 
transfer to the final drawing by means of tracings, which are 
oriented with reference to the plotted triangulation points and 
previously located camera stations, all the different points 
obtained by intersections upon the construction board, in order 
to erase therefrom all lead-pencil lines which served for their 
determination, to obscure as little as possible subsequent con- 
structions for the location of the positions of other points of the 
terrene. 



284 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



III. L. P. Paganini's Graphic Hypsometer (" Squadro grafico "). 

After the plotting of the positions of the more important 
secondary and tertiary points, in the horizontal sense, is well 
under way, it remains to ascertain the elevations of the various 
stations, including the secondary and tertiary points of the per- 
spectives, in order to enable the draughtsman to interpolate the 
contours between the plotted points that control the terrene 
forms of the area to be mapped. 

The elevations of all plotted camera stations may be ascer- 
tained by aid of Paganini's graphic hypsometer, Fig. 167, Plate 
LXXXIX, using the plotted distances between the camera sta- 
tions and surrounding triangulation points and the correspond- 
ing angles of elevation, which had been observed to the latter 
from the camera stations and which are recorded in the field 
note-books (Model No. I, Chapter VIII). 

The elevations of all secondary and tertiary points may be 
determined with the same instrument by means of their graph- 
ically measured distances from the camera stations and their 
corresponding ordinates {y) measured on the perspectives. 

Two rulers LU and MM', Fig. 167, Plate LXXXIX, may 
be made to glide with their ends U and M' along a ruler AB, 
always maintaining a position perpendicular to the latter, how- 
ever, for which purpose their ends are secured to two sleighs L" 
and M" which fit into two parallel grooves g and g'. The 
motion of LL', or rather L", is free, and it is accomplished by 
pushing the button up or down the ruler AB. 

M" is provided with a ratchet and pinion P. By turning 
the latter in one direction or in the other the ruler MM' will 
be gradually moved up or down AB, as the latter is provided 
with a row of teeth into which the ratchet-wheel of M" bites 
while P is being revolved. 

The alidade ruler d'd is secured with one end, d, in V in such 
a manner that dd' may be freely revolved about the axis of V 



L. P. PAGANINl'S GRAPHIC HYPSOMETER. 285 

as a center, while the other end, d', passes over a graduated 
arc Ggg'. The plug in V is similarly constructed as the one in V 
of the graphical sector, Fig. i66, Plate LXXXVIII (it is pro- 
vided with a revolvable button containing a slot, in such a man- 
ner that the ruler AB may be revolved simultaneously with 
the alidade d'd about a needle, marking the station point on the 
construction board). In this instrument the plug, the revolvable 
button, and the alidade ruler dd' have each a slot which inter- 
sect each other in the center of rotation V, and through which the 
needle marking V may be passed when they all have a certain 
position, the needle being again secured in place by a quarter- 
turn of the button. The entire instrument may be revolved 
about the needle in V-. the center of which lies in the directions 
of the fiducial edges of the ruler AB and aUdade dd'. 

The alidade ruler dd' is provided with a vernier n, graduated 
to read to 30 seconds on the graduation of the arc Ggg'. This 
vernier serves to lay off angles from V included between the 
fiducial edges oi AB and dd'. When dd' is brought close to 
and in contact with AB, the zero of the vernier n and the zero 
of the arc graduation Ggg' will coincide. The axis of the instru- 
ment is represented by that edge oi AB (facing dd') which passes 
through the center of rotation V; and which passes through 
the zero of the graduated arc Ggg'; it also passes through the 
point p of the line pc[, which is marked upon the ruler MM', and 
it is provided with a millimeter graduation. This line pq cor- 
responds with the zero of the vernier n', which is attached to 
MM' and which glides along AB with MM' when the latter 
is moved up or down the ruler ^5. AB \s, provided with a milli- 
meter graduation also, and by means of the vernier n' the dis- 
tance ^F of the line pq from the center of V may be read to o.i mm. 

When the line pq is brought to the distance = / from V, by 
means of the fine ratchet movement at P, the line pq may be 
regarded as the axis of abscissae drawn upon the perspective, 
while the point p will then represent the principal point of the 
perspective (see Fig. 168). 



286 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

In this case the line pc[ may also be regarded as the axis of 
ordinates of the perspective mn, Fig. 'i 68, Plate LXXXX, pro- 
vided the principal plane (containing VP and the axis of ordinates) 
is supposed to have been rotated about VP until it coincides 
with the horizontal plane VPO'. 

The point p is permanently marked upon the line pq (in 
the same way as described for the graphic sector) by a small 
puncture, which likewise serves to receive one point of the dividers, 
when such are used to lay off the coordinates, taken from the 
perspective. 

After pq has been secured, at a distance = /, from the center V 
and the abscissa x of a point a, taken from the perspective mn. 
Fig. 1 68, Plate LXXXX, has been transferred to the line pq 
from p, the second point of the dividers, upon pq, will represent 
the horizontal projection a' of the point a. If we now move 
the alidade dd' until its fiducial edge touches the second point 
of the dividers, the triangle formed by the edge of the alidade 
d'd, the edge of the ruler AB, and the line a'p will represent 
the horizontal triangle. J'i^a' of Fig. i68, Plate LXXXX. 

The end d' of the alidade is provided with a steel index mark i, 
which may be moved along dd' by means of a revolvable but- 
ton, E, ending in a ratchet-wheel below, which rotates in a row 
of teeth attached to one side of the groove s's. If this index 
mark i is moved to a' (the intersection of the fiducial edge of 
the alidade dd' and line pq), the distance Va' (cut off on dd') 
will represent the horizontal distance of the point a' (of the per- 
spective mn) from V (i.e., the value d in Fig. i68, Plate LXXXX) 
measured on the scale with which the fiducial edge of dd' is pro- 
vided. Maintaining the index mark i (Fig. 167, Plate LXXXIX) 
in this position on dd' and revolving d'd about V, until its fiducial 
edge coincides with the ^dge pV oiAB, then moving the ruler MM' 
away from V (by turning the button P) until the line pq coincides 
with the index mark i, we will have transferred the distance d 
(Fig. 168, Plate LXXXX) upon the axis of the hypsometer; we 
will also have brought the line pq (engraved upon MM') to a dis- 



287- 

tance equal to d from the center of rotation in V, and by trans- 
ferring the ordinate y (Fig. 168, Plate LXXXX), measured on 
the perspective mn with a pair of dividers, upon the line pq (while 
the latter is still in the position just described), by inserting one 
point of the dividers into the cavity p and bringing the fiducial 
edge of the alidade dd' gently into contact with the other point of 
the dividers, resting on the line pq at a' (Fig. 167, Plate LXXXIX), 
then the triangle Vpa' of the hypsometer will also represent the 
vertical triangle Va'a of Fig. 168, Plate LXXXX, except that 
now it has been revolved about Va' into the horizontal plan. 

The movable ruler LL', which also remains always perpen- 
dicular to the hypsometer axis {Vp) like MM', consists of two 
plates firmly joined together at their ends, between which the 
alidade d'd may freely glide when revolved about V. The upper 
plate of LL' is slotted like the handle of a penknife and the edges 
LI and LI' are beveled and provided with a millimeter gradu- 
ation, the numerals of which correspond with a scale of 

'^ 50000 

(50 m. = 1 mm.). A ratchet-screw c serves to move a plate 
KOK', with two index marks K and K' that may be adjusted 
to coincide with the intersections of the fiducial edge of 
the alidade dd' and the two graduated and beveled edges LI 
and LV. The index plate KOK' has a double vernier, n", 

on the opposite side of the ratchet-screw c, graduated to read — 

millimeter (i.e., to read single meters for the scale) in 

connection with the millimeter scales LI and LV. 

When the zeros of the double vernier n" coincide with the 
zeros of the graduated edges LI and LI', the marks K and K' 
of the double index will coincide with the edge Vp oi AB (i.e., 
with the axis of the instrument) and also with the fiducial edge 
of the alidade d'd, the zero of the vernier n also coinciding with 
the zero of the arc graduation Gg^ (i.e., the fiducial edge of 
dd' will fall together with the axis pV oi the instrument). 



288 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

In Fig. 1 68, Plate LXXXX, A may represent a point of the 
terrene the image of which is designated in the perspective mn 
by a. If A' be the orthogonal projection of A in the horizontal 
plane passing through the second nodal point V, then A A' 
will represent the difference of elevation =X between the points 
A and V. A'V will be the horizontal distance =Z? of the point A 

from the camera station V, which distance is represented by 

^ 50000 

for a scale of map of 

^ 50000 

Returning to Fig. 167, Plate LXXXIX, we now imagine the 
hypsometer revolved about the needle center in V until the 
hypsometer axis pV passes through a plotted point A' in the 
drawing. If the ruler MM' had previously been secured in 
such a position that the distance pVoi p from center of station V 
is equal to d and if dd' had been set to lay off the ordinate y 
upon pq from p, and if we now bring the index mark if in a 
position to mark the intersection of the fiducial edge of the ali- 
dade dd' with the edge LI of LL', then the triangle VAA', Fig. 167, 
Plate LXXXIX, will also represent (in the . scale of i : 50000) 
the triangle VA'A of Fig. 168, Plate LXXXX. 

The index mark K indicating, on the beveled graduated 

edge LI, the length , we find the difference of elevation 

between the point A and camera station V by reading the cor- 
responding vernier of the double vernier n". 

The triangles Vpa' and VA'A (Fig. 167, Plate LXXXIX) 
being similar, we will have 





AA' Pa' y 
VA'~Vp~d 


We know that 






y L 
d D' 



hence 



we have 



I. p. PAGANINl'S GRAPHIC HYPSOMETER. 289 



AA' L D 



AA 



50000 
L 



50000' 
£ = 50000X^4^'. 

The numerals of the graduation of the edges LI and LV and 
of the double vernier- n" give the value AA' already multiplied 
by 50000, which is the true difference of elevation. 

With reference to Fig. i68, Plate LXXXX, we have 

L y AA' 
tan a=-^=- 



D d VA'- 

Hence if we know the angle of elevation of a point A of the 
terrene we need only to lay ofE this an le upon the graduated 
arc Ggg' by means of the alidade vernier n, from g and place 
the index mark K upon the intersection of the fiducial edge of 
the alidade dd' and edge LI (the instrument having been placed 
upon the plotting-sheet in such a position that the hypsometer 
axis pV passes through the station V and the plotted point A'), 
and then read off on LI and corresponding vernier n" the differ- 
ence of elevation between camera station and point A. 

This case becomes very much simplified when the image A' 
of A is bisected by the principal line of the perspective (axis 
of ordinates), as then 

x = o and d = f. 

The aUdade dd' is placed so as to lay off the ordinate of the 
point a upon pq from p, after the ruler MM' had been secured 
in a position at a distance = / from V; then the index mark K 



■290 PHOTOTOPOGKAPHIC METHODS AND INSTRUMENTS. 

or K' is brought into the point of intersection of the fiducial 
edge of dd', with the edge LI or LI' of the ruler LL' (the axis of 
the hypsometer passing through the plotted point A'), and the 
difference of elevation between A and V is read ofif, either on 
the vernier corresponding to the graduation LI, or on that cor- 
responding to the graduation LI'. 

The corrections for curvature and refraction, to be applied 
to these differences of elevation, are taken from the ordinary 
field tables. 

IV. The Centro-linead as Used by Capt. E. Deville. 

Reference to this instrument has already been made under 
the head of Photograph-board, Chapter VI; it is used in Canada 
under Capt. E. Deville. 

We had seen that the distance between the principal point 
and the vanishing points of lines increases the nearer parallel 
to the picture plane such lines would become. Lines parallel 
to the picture plane have their vanishing points at infinite dis- 
tance from the principal point; practically they have no vanish- 
ing points. Their perspectives are parallel with the original 
lines. 

In iconometric plotting it frequently occurs that the vanish- 
ing points of some lines fall outside of the limits of the drawing 
board, and in order to draw a line which if produced would 
pass through the distant vanishing point, special constructions 
would have to be made to locate the direction of such a line. 

To avoid making such auxiliary constructions on the photo- 
graph board Capt. Deville uses the so-called " centro-linead," 
with which a line vanishing at any distant point may be drawn 
upon the picture plane no matter how far off from the principal 
point of the perspective the vanishing point may be. 

This instrument, represented in Fig. 169, Plate LXXXX, 
is composed of a straight edge L (of wood) and two wooden 
movable arms / and V. The inclination of these arms / and /' 



THE CENTRO-LINEAD AS USED BY CAPT. E. DEVILLE. 29 1 

against the straight edge may be varied to any angle. The arms 
may also be permanently fixed in any position by means of the 
clamp-screws r and r" . 

We had seen that the photograph-board, Fig. 68, Plate XXXV, 
was provided with four points ABC and £, indicating the centers 
of the studs against which the arms / and /' of the centro-linead 
play or rest when the same is used on the photograph-board. 
The distance between these studs may vary. Each two forming 
a pair are generally placed from six to eight inches apart, and 
the arms of the centro-linead being held in contact with the 
studs, the various directions of the ruler L will intersect each 
other in one common point. 

Referring to Fig. 170, Plate XCI, 

A and B may represent one pair of studs, fixed upon the board ; 
OA and OB the arms of the centro-linead, clamped in the posi- 
tions indicated, and 
OC the ruler of the centro-linead. 

If we describe a circle through the three points A, O, and B — 
the angle AOB remaining constant — the angle AOB will be 
an angle of the periphery AB for any position given the ruler L 
(or OC) as long as / and I' {OA and OB) remain in contact 
with A and B (the two studs on the periphery of the circle). 
When OC is changed to the position O'C the intersection V 
of the two hnes OC and O'C will also be on the periphery of 
the circle because the angle AOV {AO'V) remains the same 
and must subtend the same arc .4 F as long as the position of 
the studs remains unchanged. Hence for the assumed position 
of the studs the directions of all lines drawn along the fiducial 
edge of the ruler L (giving O all positions on the arc AOB) 
will pass through the point V; they will vanish at V. 

In the iconometric work of the Canadian surveys the centro- 
linead is used only for drawing the perspectives of horizontal 
lines whose vanishing point is on the horizon line. The studs 
A and B are placed on the photograph-board on a line AB per- 



292 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

pendicular to the horizon line and at equal distances from the 
latter. The horizon line {DD', Fig. 68, Plate XXXV) HH', 
Fig. 170, Plate XCI, becomes a diameter of the circle AOBV 
and VA = VB. 

If the arms I and I' of the centro-linead include the same 
angles with the ruler L, the line OC, bisecting the angle AOB, 
must pass through V midway between A and B. 

The distance of the vanishing point V from the principal 
point P may be varied at pleasure by changing the inclination 
of the arms I and V against the ruler L. When the directions of the 
arms I and V fall together and are perpendicular to L, the vanish- 
ing point will fall at infinite distance from the principal point P, 
and the lines drawn along the ruler L will be parallel to the horizon 
line. 

The distance of the vanishing point V from P may also be 
varied by changing the distance between the studs A and B, 
or C and E, Fig. 68, Plate XXXV; increasing this distance 
enlarges the circle 4 OJBF and V moves farther off from P; reduc- 
ing that distance decreases the diameter of the circle AOBV 
and V will approach the principal point P. The practice in 
Canada, however, is to retain the position of the studs unchanged 
on the photograph-board and to change the inclination of the 
arms / and /' of the centro-linead instead. 

If we gradually close the arms I and I', V will approach the 
line AB, and when the angle AOB becomes equal to 90° the 
arc AOB will have become a semicircle and the intersection of 
AB with HH' will be the center of the circle AOBV, the distance 

AB 

of both O and V from AB will be equal to ; continuing to 

close the arms / and /', V will approach closer to AB without 
ever reaching it. 



THE CENTRO-LINEAD AS USED BY CAPT. E. DEVIILE. 293 

A. To Set the Arms I and I' of the Centro-linead if the Direction 
to the Vanishing Point be given by a Line in the Ground 
Plan. 

In Fig. 171, Plate XCI, 

P= principal point on the photograph -board ; 
A and 5 = positions of the two studs; 

5^1= given direction of the line on the ground plan, when V will 
be the vanishing point for that line. 

If we revolve the picture plane about the horizon line as axis 
into the horizontal plane the station may fall in S, Fig. 171, 
Plate XCI, when SP will represent the horizontal projection 
of the principal ray or the distance line (focal length) of the 
picture. If the point V would fall upon the drawing-board we 
could describe a circle through AB and V and place the fiducial 
edge of the ruler upon DP (the horizon line) with the axis of 
rotation of the arms I and I' m D upon the circle, bring the 
arms I and /' into contact with the studs A and B and clamp 
them in that position. In this case there would be no use for 
the centro-linead, however, as F is accessible. 

If we join VB, the angle VDB — the inclination of the lower 
arm I' against the ruler L — is equal to VBA, both angles sub- 
tending equal arcs of the same circle. Draw the lines CS and 
BS. At any point c on CS draw cM and cv parallel to AB 
and DP and join b and v. By reason of similarity of triangles 
vb must be parallel to VB and the angle 

vbc = VBC=BDV. 

Hence the arms of the centro-linead may be set, in the case 
under consideration, by placing the ruler L on Mb, the axis of 
rotation, 0, coinciding with b and adjusting the lower arm /' 
of the centro-linead to coincide with bv. The other arm /, having 
the same inclination against the ruler L as I', may be set by placing 



294 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the ruler L upon the horizon line DP and moving it along this 
line until the lower adjusted arm /' comes into contact with the 
stud B, then, moving the other arm / about O until it comes into 
contact with the stud A and clamping it also. 

The lines BS, CS, Mc, and cv are drawn once for all upon 
the photograph-board, Fig. 68, Plate XXXV. The only line 
to be drawn for setting the centro-linead arms is Sv, which is 
the direction of the given line on the ground plan. The line bv 
need not be drawn, the points h and v being located by draw- 
ing cv parallel with the horizon line and cM or ch parallel to the 
distance line SP. 

B. To Set the Arms of the Centro-linead ij the given Line {VE) 
belongs to the Perspective. 

Take any point F, Fig. 172, Plate XCI, on the horizon line, 
join F with E and F with B, then draw cM parallel to AB. 
Through e draw ev parallel to EV and join vb. Owing to the 
similarity of triangles vh will be parallel to VB and the angle 

vbc = VBA, 

which is the inclination of the arm against the ruler L of the 
centro-linead. The lines FB and cM are permanently laid 
down on the photograph-board. Fig. 68, Plate XXXV, but FE 
and ve will have to be drawn for every given line; in this case 
two Hnes will have to be drawn instead of one as in the pre- 
ceding case. 

Centro-lineads are usually sold in pairs; one serves to work 
on the left side of the principal point and the other on the right. 



V. The Perspectometer as Used by Capt. E, Deville. 

The perspectometer serves to dispense with the con- 
struction of the squares on the perspective, when using the 
method of squares (Chapter IV), to draw a figure in the 



THE PERSPECTOMETER AS USED BY CAPT. E. DEVILLE. 295 

ground plane by means of its perspective. On a thin trans- 
parent film (glass, xylonite, isinglass, horn, etc.) two parallel 
lines AB and DD', Fig. 173, Plate XCII, are drawn intersecting 
the common perpendicular pP. Make DP = PD' =pA=pB=dis- 
tance line (focal length) and from p lay off on AB (to both sides 
of p) equal distances, 

pm = mn = no . . . =p'm' = m'n' =n'o' . . , . 

Join these points of division to P and through the corresponding 
intersections of the radials from P with AD and BD' draw lines 
rr', ttf . . . , which lines wiU be parallel to AB and DD'. 

Use of the Perspectometer. — The instrument is placed upon a 
perspective with P on the principal point and DD' coinciding 
with the horizon line. The ground line of the perspective may 
fall in XY; it is divided into equal parts by the radials from P, 
and the trapezoids of the perspectometer represent the per- 
spectives of the squares in the groimd plane having the equal 
parts of XF as sides. 

By placing the perspectometer on the perspective in the 
manner indicated above, the squares covering the perspective 
of the figure, which is to be plotted iconometrically on the ground 
plan, are at once apparent and only those needed are drawn 
on the ground plan. 

The sides of the squares to be drawn on the ground plan 
(their side lengths are equal to the divisions on the ground hne 
between the radials trom P) are laid off from the trace of the prin- 
cipal plane on the ground line, and the position of the front Une 
nearest the picture trace (or ground line) is laid off on the ground 
plan either by estimation or by construction. The estimation 
of the position of this line (corresponding to tf) on the ground 
plane is made by noting the fraction of a square's side which 
represents the distance (between tf and XY, Fig. 173, Plate XCII) 
from the groimd line on the perspective. 

The perspectometer serves only for perspectives which have 



296 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the same distance line (like photographs of distant objects taken 
with the same lens), different distance lines requiring a new 
perspectometer to be constructed for every one. 

The width pP should be equal to the height of the horizon 
line above the foot of the picture; the radials need not extend 
beyond the width of the picture, the distance points D and Z)' 
having been taken as the limit of the perspectometer in the 
figure (Fig. 173, Plate XCII) merely to show more fully the 
principles involved in its construction. The length of a single 
division on the line AB should be selected with reference to the 
resultant equal division lengths of the lowest ground line used 
for the pictures, as the divisions on the latter give the measure 
for the sides of the squares to be drawn on the ground plan. 

These division lengths on the ground line should be in har- 
mony with the scale of the plan and with the degree of accuracy 
that may be required for the delineation of the topographic 
features. The smaller the size of the squares on the ground 
plan selected the more accurately the transfer to the ground 
plan of the figure from its perspective may be made; the same 
principles being involved, in this method of iconometric plotting 
as in the well-known method of reducing (or enlarging) draw- 
ings by means of two sets of squares, the ratio of their sides 
corresponding to the scale of the required reduction (or en- 
largement). 

Capt. Deville recommends the perspectometer to be made 
by first drawing it on paper in a fairly large scale and then 
making a negative of it, reduced photographically to the desired 
size of the finished perspectometer. A positive copy of said 
negative may then be made on a transparency plate, which, 
if bleached in a solution of bichloride of mercury, will show 
white lines (they were dark before the bleaching took effect) 
on clear glass. For the sake of better preservation the per- 
spectometer is varnished when completely dry and hard. 

When using the perspectometer to transfer figures from 
their perspectives to the ground plan, when such figures are 



THE PERSPECTOMETER AS USED BY CAPT. E. DEVILLE. 297 

situated in planes perpendicular to the picture plane but in- 
clined against the horizon plane, the center of the perspec- 
tometer is placed upon the principal point P of the picture 
plane the same as before, but the perspectometer is now re- 
volved about P until the parallel lines of the same are parallel 
to the trace on the picture plane of the inclined plane (contain- 
ing the figure to be transferred). In this case the trapezoids 
of the perspectometer represent the perspective of a net in the 
inclined plane composed of squares which are now to be pro- 
jected into the ground plane. 

This net of squares in the inclined plane, when projected 
on the groimd plan, will be composed of rectangular figures 
of equal size, their long sides being in a direction at right angles 
to the picture trace (or ground line) and of a length equal to 
that which is intercepted between two adjoining radials of the 
perspectometer on the trace of the inclined plane (on the pic- 
ture plane), while the short sides of those rectangles forming 
the net of the ground plan will be equal to the lengths obtained 
on the ground line by projecting the points of intersection of 
the radials of the perspectometer with the inclined plane's 
trace on the picture plane upon the ground line of the picture 
plane. 

The construction of the rectangular net on the ground plan 
may now be made in an analogous manner to that mentioned 
for the squares, and the drawing-in of the figure on the ground 
plan with reference to its position within the trapezoids of the 
perspectometer is accomplished in the usual manner. 

Should the figure be situated in planes that are inclined 
to both the picture and the groimd planes, then the figure is 
first projected upon a plane perpendicular to the picture plane 
and having the same trace in the latter as the inclined plane. 



298 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



VI. The Perspectograph, Devised by H. Ritter. 

Numerous instruments have been devised for the mechanical 
drawing of perspectives from plans (or from nature), or by 
means of optical devices, some of which may, inversely, be- 
come of use for transcribing perspectives of figures into orthog- 
onal projections, and we have seen that Col. Laussedat as far 
back as 1849 made his first experimental perspective surveys 
with the camera lucida or camera clara, devised by Wallaston, 
which in this case had been improved by Regnault. The per- 
spectograph invented by H. Ritter serves to construct the 
orthogonal projection of a plane figure from its perspective 
or to draw the perspective from the plans of the object without 
referring to the object itself. 

Ritter's instrument, manufactured by C. Schroeder & Co. 
in Frankfort on the Main, has been patented in Germany, 
Oct. 13, 1883, under No. 29002. It was devised primarily 
for architectural purposes. For the title of Ritter's descriptive 
pamphlet, see Literature, paragraph 2, Chapter I. 

This instrument in its present form, composed largely of 
wood, is not well suited for surveying purposes, as it contains too 
many sources of error, due to lost motion in its bearings; still, 
its theory being a good one, there is no reason to doubt its ulti- 
mate value, even for precise work, if it were carefully made by 
an expert mechanician, excluding the use of wood and using 
metal throughout, being guided in its construction by the de- 
mands of the utmost attainable precision. Since a carefully 
constructed instrument based on the present pattern may become 
useful in plotting the data of a topographic reconnaissance, 
where, in the nature of the work, rapidity in making the results 
practically available will often be of greater value than a high 
degree of accuracy, the following description of this instrument 
may not be out of place here. For its methods of use in photo- 



THE PERSPECTOGRAPH, DEVISED BY H. RITTER. 299 

topographic surveying we respectfully refer to Capt. Deville's 
work on photographic surveying. 

We have seen that the plotted position in the ground plan of 
a point may be found from its perspective by locating the inter- 
section of the horizontal projection of the ray: "station pic- 
tured point" with the Une of direction found by revolving this 
ray with its vertical plane iiito the ground plane (about the trace 
of the vertical plane in the ground plane as axis of revolution). 

With reference to Fig. 174, Plate XCII, 

S may represent the camera station; 
M the position of a point plotted on the ground plan GG; 

H its perspective in the vertical picture plane MN; 

s the foot of the station S; 
XY the ground line of the picture plane MN. 

If we draw through the foot of the station a line parallel to 
the ground line XY and make its length, s{S), equal to sS, join 
the plotted point M with (5), then it will follow, from the simi- 
larity of the triangles O/tW and sSM, that 

sS:0(i=Ms:MO. 

The triangles s{S)M and 0(fi)M being also similar, we find 

s{S):Oifi)=Ms:MO; 
hence 

sS:0;i = s{S):0(fi). 

As we had made sS=s(S), the last equation can only prevail if 

Ofi = 0{/i). 

To find, therefore, the perspective, fi, of a point, M, given on 
the ground plan, we first draw through the plotted station, on 
the ground plan, a line s{S) parallel to the ground line XY, 
making 5(5)= height of the station 5 above the ground plane. 
Draw the lines sM and {S)M, which will intersect the ground 
•ine, XF; in O and (/x), Fig. 175, Plate XCIII. On the ground 



300 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

line X'Y', drawn in another place of the working-sheet, we assume 
a point O', representing of the ground plan, and erect opt per- 
pendicular to A'' I'' in O' and equal to 0(fi), when // will be the 
perspective of M in the reverse position of the perspective. The 
perspective of any other point Ai^ on the ground plan may be 
found in the same way, making 0'Q'=OQ and Q'v = Q{u). 

Ritter devised the perspectograph with reference to the pre- 
ceding relation between the visual ray, SM, Fig. 1 74, Plate XCII, 
to a point M, the horizontal projection of the ray, and the plotted 
position of such point M, the perspectograph performing the 
preceding construction. Fig. 175, Plate XCIII, mechanically. 

The general arrangement of this instrument is shown in Fig. 
176, Plate XCIII: sM and {S)M are two slotted wooden arms 
carrying the tracer, M, at their point of intersection. The con- 
nections at 5, 0, (s), and (/() are such that the rulers sM and {S)M 
may slide through these points. The slide connections 5 and 
(S) may be moved along the groove or slot of the wooden ruler 
RT. The sliding piece O is secured to a rod which may slide in 
the groove shown in the wooden ruler XY, being connected at 
its other end D with a system of arms, joined together after the 
manner of a pantograph. The distance OD is maintained 
unchanged while the instrument is in use. 

The center of s is placed over the point which marks the 
plotted camera station on the ground plan, and the ruler RT is 
placed parallel to the ground line of the picture plane, s and 
RT are then secured in this position on the ground plan. 

When the arm sM is moved, s being held in a fixed position, 
the point wiU follow the motions of the arm sM, also applying 
its motion directly to the arm OD (which slides in the groove of 
XY) and indirectly to the arms of the pantograph system. 

The fourth sliding piece (n) is connected with the point A of 
the pantograph system by means of a separate piece which insures 
a permanent distance between (fi) and A while the instrument is 
in use, and which may slide on the rod OD. The pantograph 
system is composed of six pieces: four straight arms, AB, AC, F/i, 



THE PERSPECTOGRAPH, DEVISED BY H. RITTER. 30 1 

and Fn', and two double arms, CDE and BDG, which are bent at 
right angles in their points of junction D. The sides of the two 
parallelograms ABDC and DGFE are all of equal lengths, and 
the six arms are joined in A, B, C, D, E, F, and G. The lengths 
•of the arms F/i and Ffi' are twice that of the side of the parallelo- 
grams. The pencil which describes the perspective may be 
attached to the free end of either arm Ffi or Ffi'. 

The angles GDB and EDC being each equal to 90°, the sum 
of the two other, angles CDB and GDE must be equal to 180°. 
The sum of two adjacent angles in a parallelogram being also 
equal to 180°, it follows that 

CDB + GDE = CDB +DCA, 

or GDE = DC A, 

which shows that the two parallelograms are also equiangular, 
and as their sides are equal in length it follows that the parallelo- 
grams themselves must be equal, but they are placed in different 
directions. The diagonals FD and GE of the one are equal to 
BC and DA of the other, respectively. The two long arms 
Fft' and F/j. being of the same length, ftfi' will be parallel to GE, 
both will be perpendicular to the direction of XY, and ju/t' will 
pass through D. We have, therefore, 

Dfi'=DiJL=GE=DA. 

Use 0} the Perspectograph. — The sliding piece 5 is secured 
to the working-board over the plotted position of the camera 
station on the ground plan, still permitting a gliding movement 
of the arm sM in the direction sM. The center line of RT is 
brought into a position parallel to the plotted ground line and 
its position is also secured to the board. The sliding piece (5), 
finally, is moved from ^ (in the groove of RT) until s{S) is equal 
to the elevation of the station 5 above the ground plane, also 
securing (5) in this position, when it will still permit a gliding 



302 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

movement of the arm {S)M in the direction of (S)M. The 
center Hne of the wooden ruler XY is placed upon the ground 
line (picture trace) on the ground plan. 

The manipulation of the instrument and its general working 
will now readily be understood. For instance, when the tracer M 
is moved in a direction parallel to RT or XY, the arm sM will 
also move the slide OD in the same direction. The distance 
0(/i) remaining unchanged as long as s(S) undergoes no change, 
{lx)A will also remain of a constant length. Hence, AD and 
also GE as well as Dft undergo no changes, and the pencil in /t 
or in // will trace a parallel line to XY; representing the perspec- 
tive of a line of the ground plan (the one traced by M) and parallel 
to the picture plane. 

When M is moved in the direction of sM, away from XY, 
the positions of O and D remain the same, but 0(ix) will be 
lengthened, (/x) moves to the right — away from O — carrying the 
point A with it {A(fi) being a constant length) and increasing 
the length of the diagonal DA in proportion to the increase of 
the length 0(fi). DA, being equal to GE, equal to Dii{ = D/j.'), 
the latter will also be lengthened and /x will move down — away 
from XY — by the same amount as (fi) is moved to the right. 
The relation between 'the construction made in Fig. 175, Plate 
XCIII, and the mechanical plotting with the perspectograph, 
Fig. 1 76, Plate XCIII, will now be evident. 

VII. Prof. G. Hauck's Trikolograph and its Use in Iconometric 

Plotting. 

This instrument has been described by Dr. G. Hauck in a 
memorial commemorating the opening of the new building of 
the Royal Technical High School at Charlottenburg, near Ber- 
lin, Nov. 2, 1884. It serves to reconstruct an object from two 
perspectives obtained from two different points of view. 

The principles which underlie the construction of this instru- 
ment hold equally good for the construction of an instrument 



THE HAUCK TRIKOLOGRAPH. 303 

which could serve to plot mechanically the ground plan of any 
object represented on two photographs obtained from different 
stations. 

Prof. F. Schiffner, in 1887, suggested the changes to be made 
to Dr. Hauck's instrument in order to render it available as an 
instrument of precision for the use of the phototopographer; still, 
it seems that mechanical difficulties in its manufacture are yet 
to be overcome, as the writer has not met with any record of 
such an instrument having been in use or even constructed. 

In Chapter IV it has been shown that a point, A, photo- 
graphed from two stations, S and S\, may be plotted in hori- 
zontal plan, if the two picture traces gg and gxgx, and the two 
camera stations S and Sx, are given on the horizontal plan. Fig. 
177, Plate XCIV. 

The two picture' planes may be revolved about their ground 
lines, gg and gxg\, into the horizontal or ground plan, when (a) 
and (fli) will be the two images of the point. A, revolved into 
the ground plane. If we draw lines through (a) and (ai) per- 
pendicular to the corresponding ground lines gg and g\g\, then 
al and a'x (Fig. 177, Plate XCIV) will be the projections of the 
pictured points a and ax into the horizontal plan and the inter- 
section of the radials drawn from 5 and Sx to a' and a/, re- 
spectively, will locate the position A' of the point A pictured 
on the two plates as a and ax- 

This graphical determination of the plotted position A' of 
the point A may be accomplished mechanically by placing 
slotted rulers with their center lines upon gg and gigi Fig. 178, 
Plate XCIV, and indicating the directions of the perpendiculars, 
dropped from the pictured points (revolved into horizontal plan) 
upon the ground lines, by two arms, (a)bc and a'h, of a panto, 
graph combination, where 

(a)h = hc = a'h. 

The points (a) a' and c will always be situated on the pe- 
riphery of a semicircle described about h as the center, and as 



304 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the points c and a' are permanently held on the line gg, the angle 
aa'c (angle of the periphery subtending the semicircle) will be 
equal to go° for all inclinations that may be given {a)c against gg. 

The directions of the radials Sa' are laid down mechan- 
ically by means of two slotted rulers Sa' and Siai, held in posi- 
tion by the studs in 5 and a' (in Si and fli'), both rulers being 
revolvable about the fixed points 5 and S\. 

This instrument, of which the characteristic features are 
shown in Fig. 178, Plate XCIV, performs the constructions 
mechanically which were made graphically or geometrically in 
Fig. 177, Plate XCIV. 

The slotted rulers gg and g\g\ are secured to the plotting- 
ioard (with their center lines on the picture traces) by means of 
thumb-tacks T. The pantograph-arms (a)c(ai) Ci and a'b ai'bi 
are connected with these rulers by means of sliding joints c 
(and Ci) and a' (and a/), while the studs which mark the sta- 
tions 5 and Si end in cylindrical projections which fit into the 
slots of the rulers Sa' and 5iOi', the latter fitting also over similar 
cylindrical attachments to a' and ai', in such a way that the 
rulers Sa' and 5ifli' may freely glide over the points S and a' (or 
Si and fli')) and at the same time may revolve about the fixed 
points 5 and Si respectively. 

The points (a) and (fli) are provided with tracers and a pencil- 
slide is attached to the intersection of the rulers Sa' and Siai 
(in ^') in such a way that the pencil point may freely slide either 
way in the grooves of Sa' and Siai'. 

A comparison between Figs. 177 and 178, Plate XCIV, will 
plainly show that A' will always represent the plotted position 
of two images (a) and (oi) (revolved into horizontal plan) of 
the identical point A. 

It may not always be possible to identify both images of the 
same point A on the two pictures, and in order to apply Prof. 
Hauck's method, to identify the second image (on the second 
photograph) by means of the so-called "kernel points" the 
instrument, shown in Fig. 178, Plate XCIV, must be modified 



THE HAUCK TRIKOLOGRAPH. 305 

in such a way that the point of the second tracer will always be 
upon the image (on the second picture) which the point of the 
first tracer designates on the first picture (revolved into the 
ground plane). 

We had seen in Chapter IV that the line connecting the 
image of any point A on the first picture with the image of the 
second station (kernel point (si), Fig. 179, Plate XCV) and 
the line connecting the image of the same point A on the se end 
picture with the image of the first station (kernel point (s), Fig. 
179, Plate XC\^) will bisect the same point a of the line of 
intersection of the two picture planes. The picture planes being 
vertical, this line of intersection will be the vertical line passing 
through the point Q of the ground plane (point of intersection 
of the two picture traces or ground lines gg and ^1^1). The 
picture planes having been revolved about their ground lines 
as axes into the horizontal plan, this Une of intersection aQ, also 
revolved into the groimd plane (and about gg and again about 
gigi)> will appear twice, once as Q(a), perpendicular to gg in fi, 
and again as Q{ai), perpendicular to ^1^1 in Q. As the points 
(a) and ((Ti) represent the same point a, revolved into the hori- 
zontal plane, once about gg and again about ^1^1 as axes, the 
lengths {a)Q and {ai)Q must be equal. 

In order, therefore, that this instrument (Fig. 178, Plate XCIV) 
may work iii harmony with the principles which underlie Prof. 
Hauck's method, it wiU have to be modified to fulfill the follow- 
ing conditions: 

A line drawn through the kernel point .^i and any point pictured 
on the first photograph, and a line drawn through the kernel 
point s and the image on the second photograph of the same 
point, are to intersect the line of intersection of both picture 
planes in the same point a, or, the two lines revolved into the 
horizontal plan (with the picture planes) must bisect the re- 
volved lines (ff)i3 and {ffi)Q in points (a) and (cti), which are 
equidistant from Q. 

The complete instrument is represented in a general way 



3o6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

in Fig. 179, Plate XCV. The two slotted rulers gg and gigi, 
of Fig. 178, Plate XCIV, have been supplied with additional 
arms Q(a) and S{ai), each arm including an angle of 90° with 
its ruler. These rectangular elbow-pieces are secured to the 
plotting-board by four thumb-facks T after the rulers gS and 
giQ had been placed with their center lines upon the picture 
traces gg and ^1^1, respectively, in such a way that the intersections 
of the center lines of the elbow-rulers, at the rectangular elbow 
end of the rulers, coincide with the intersection 8 of the ground 
lines or picture traces gg and gigi. The pantograph-arms, repre- 
senting the ground lines of the pictures, are attached to the rulers 
the same as in Fig. 178, Plate XCIV. Studs are inserted into 
the kernel points (si) and (s), and the arms 8(a) and S(ai) sup- 
port a ruler ic)(ai), which may glide freely over these arms 
of the elbow-pieces. To cut off equal lengths on the elbow- 
arms Q(a) and 8(ai) by this ruler {a)(ai) the angle d(a)e is ad- 
justable, and it should be regulated for each set of two picture 
traces to make 

(a)Q = (ai)Q. 

When ((T)d is moved along the slot of (a) 8 the slide point 
((Ti) will move along {ai)8, 8{a) always being equal to 8{ai). 
The screw d serves to clamp the angle d{a)e for any opening 
corresponding to the angle g8gi included between the picture 
traces. Slotted rulers are now placed over the studs marking 
the kernel points (51) and (5), the slots also receiving the cylin- 
drical prolongations of the tracers (a) and (ai) and those of the 
sUde points {a) and (ai) respectively. Finally two slotted 
rulers RS and RySi are placed over the studs 5 and 5i (they 
mark the plotted positions of the two stations) and over the 
sliding joints a' and ai' (which are the same as those in Fig. 178, 
Plate XCIV). At their point of intersection. A', the sliding 
pencil point is inserted into the slots, and this completes the 
instrument. If we now move the tracer (a) on the first photo- 
graph, the pantograph arms {a)c and ha' will change the position 



THE HAUCK TRIKOLOGRAPH. 307 

of the ruler SR into the direction of the radial from 5 to the hori- 
zontal projection — on the picture trace — of the pictured point 
designated by the tracer point (a) on the first photograph and 
the ruler (a){s) is moved, locating the point (a). This change 
in the position of (a) produces a corresponding change in the 
sliding point (ai), which in turn changes the position of the tracer 
(ai), causing the pantograph- arms (ai)c and JiOi' to move, and 
a change in the position of ai will cause the radial ruler i?i5i 
to assume a new position also and the intersection of RS with 
the new position of J?i5i locates the plotted position in hori- 
zontal plan of the point under the tracer on the first photo- 
graph without actually having identified the corresponding 
image as the identical point under the tracer (ai) on the second 
picture. 

If a hne on either photograph is followed out by one of the 
tracers (a) or (ai), the pencil point A' will draw the horizontal 
projection of the pictured line, the second tracer being watched 
merely for the sake of obtaining a check or to aid its course, 
if necessary, by a gentle tapping, when the movements of the 
various parts of this instrument should retard its motion owing 
to too much friction or lost motion. 

Until now no perfect perspectograph has been constructed, 
and no matter how accurately such instruments — like the one 
just described — ^may be made by the mechanician, there will 
always remain some unavoidable imperfections in the piaterial 
or in the workmanship of the instrument, producing more or 
less error in the results. For accurate and precise work, there- 
fore, all iconometric plotting (when applying the radial or so- 
called plane-table method) should be accomplished with the 
aid of graphical or geometrical constructions, at least for all con- 
trol points of the survey, relegating the use of perspective instru- 
ments to the filling in of such details, which in an instrumental 
survey of like character would be sketched by the topographer. 



308 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



VIII. The Carl Zeiss Stereoscopic Telemeter and the Stereo- 
comparator, including the Stereophotogrammetric Survey- 
ing Method, Devised by Dr. C. Pulfrich. 

Stereoscopic surveying, when employed for phototopography, 
has many advantages, especially if the stereoscopic views of 
the terrene may be transferred into the orthogonal horizontal 
projection of the plan or map by means of stereoplanigraphs, 
or stereoscopes that are supplied with the necessary details 
and means for adjustment that may be required, for the semi- 
mechanical plotting of topographic control points. 

The idea of using two stereoscopic views of the ground, ob- 
tained from two properly selected stations, in a specially devised 
stereoscope and projecting the selected characteristic terrene 
points of the stereoscopic image directly on the plotting-shect, 
by means of a movable projecting index mark, occurred to 
Capt. Deville about ten years ago. Owing to the pressure of 
other official duties, however, Capt. Deville had to suspend the 
continuance of his exp(iriments in this direction. This inter- 
ruption is greatly to be regretted, as he had practically solved 
the problem of stereoscopic plotting by using a modification 
of the Wheatstone stereoscope. A description of Capt. Deville's 
interesting instrument may be. found in: 

Transactions of the Royal Society of Canada, Second Series, 1902-1903, 
Vol. VIII, Section III, " On the Use of the Wheatstone Stereoscope in 
Photographic Surveying.'' E. Deville. 

Also in 

A. Latjssedat. "Recherches sur les Instnunents, les M^thodes et le Dessin 
topographiques." Tome II. Paris, 1903. "La St&doscopie appliqu& 
k la Construction des Plans." 

Dr. C. PuLFEicH. "Uebcr cine neue Art der Herstellung topographischer 
Karten und ueber einen hierfuer beslimmten Stereoplanigraphen." 
Zeitschrift fuer Instrumentcnkunde, Heft V (Mai), 1903, XXIII Jahrg. 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 309 

Dr. Pulfrich has devised a stereoplanigraph which is being 
made by the Carl Zeiss firm in Jena, a description of which may 
be found in the last-mentioned paper by Dr. Pulfrich. This 
instrument seems to be planned on the lines suggested by Capt. 
Deville. 

A perfected stereoplanigraph would be the ideal instrument 
for the rapid plotting of topographic features and details if the 
terrene is controlled by a close network of triangulation. 

A. The Stereoscopic Telemeter, or Range-finder. 

The stereoscopic telemeter, or aerial distance measure, 
manufactured by the Carl Zeiss Optical Works in Jena, Ger- 
many, was first brought to general notice in a lecture delivered 
by Dr. C. Pulfrich before the Society for Natural Research, 
Munich (Sept. 19, 1899). 

This telemeter, devised by Dr. Pulfrich, is the outgrowth of 
ideas that had been suggested in a measure by Prof. Porro to 
break the straight course of the light-rays in a telescope, by means 
of a series of prisms, into a zigzag path and thus reduce the length 
of the ordinary telescope. 

The Carl Zeiss Optical firm not only succeeded to improve 
on the quality of the prism telescopes heretofore in use, but it 
succeeded also to combine two such telescopes into a binocular 
set. The relief effect produced by the Zeiss prism, binoculars, 
based on the difference between the two retinal images, is ac- 
centuated by an optical increase of the iiiterocular distance, 
simply by setting the two objectives of the binoculars farther 
apart. The ratio between the ocular and the objective distance 
gives the "stereoscopic power" of these stereobinoculars. 

The great practical success of this combination, however, 
is mainly due to the recent discoveries made in the optically 
worked glass compositions produced by the now world-famed 
Jena Optical Glass Works. Dr. Pulfrich could now realize 
H. Grousillier's idea of the aerial distance scale, and aided by 



3IO PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

the excellence of the mechanical equipment of the Carl Zeiss 
firm, the present form of the "stereotelemeter" has been manu- 
factured and placed on the market. 

With this portable stereoscopic telemeter distances may be 
read off directly, the degree of accuracy attainable in the meas- 
ures being almost entirely independent of the shapes of the 
objects determined, which, furthermore, may be stationary or in 
motion. A special transverse scale is also provided for measur- 
ing the width or length and the height of any distant object, for 
making measurements in "frontal planes." 

The Carl Zeiss firm has placed three distinct types or grades 
of stereotelemeters on the market, differing in range, magnifi- 
cation, and weight, and, of course, also in price. 

The so-called " total relief effect " may be expressed by the 

, EXG 
product , 

where £= distance between objectives ( = 510 mm.); 
c= distance between eyepieces (= 65 mm.); 
G= magnification (= 8.). 

The middle-size telemeter, to which the figures just given refer, 
will have a total relief effect of 63. That is to say, if differences 
in relief on the single plate are not observable beyond 450 meters, 
the stereoscopic image, as it appears to the observer through 
this stereotelemeter, will show differences in depth or relief 
at 63X450 m. = 28.3 km. This, however, does not mean that 
any such distances may be read with its aerial distance scale; 
it simply gives the extreme limit for recognizing terrene forms, 
all points beyond that distance appearing as infinitely far off. 
. If we direct the stereotelemeter to a point P at infinite dis- 
tance (Plate CIX) the component images of the point P will 
be at p and ^. If we now consider a second point P', just in 
front of P, its image will still coincide with p in the left image 
plane, but in the image plane of the right binocular tube it will 
appear at p", to one side of f. 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 3II 

The distance ff, spoken of as the linear parallax of the two 
points P and P', is directly proportional to the distance between 
the two points. The rays o'f and o'^' include the angle of 
parallax =5, and as the triangles o'p'p^' and P'OO' are similar 
we will have the proportion 

where /=focal length of o; 

£=interobjective distance, or telemeter base; 
£>= distance of point P from O, PP' being negligible in com- 
parison with OP. 

Hence the linear parallax 

E and / being constants, we find by differentiation 

D 

dD= da, 

a 

and substituting the above value for a we find 

dD = — r=-Yda. 
Exf 

The error in linear parallax, da, is directly proportional 
to the product of the focal length and the angular parallax 3, 
and inversely proportional to the magnification G. 

}Xd GXda 



and we may now write 



P2 . 
^^=-EXG^- 



312 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

If we now designate by r the range of stereoscopic vision 
by unaided eyes — in other words, if r is that distance at which 
an object must be placed to be seen under an angle of parallax = 5 
— we will have the relation 

r 

As d will always be a small angle we can substitute the tangent 
for the angle. 

If we designate by R the range of the field that is con- 
trolled by the total effect of relief, we will have 

E ^ EXG 

R=rX—XG 5—. 

e d 

After substitution of this value in the above equation for dD we 
finally find 

Numerous experiments have shown that the angular parallax 
(8), the angle under which objects situated in different but very 
distant frontal planes cease to appear to be at different depths 
when examined under binocular vision, amounts to 30 seconds 
for normal eyes (Helmholtz gives d = i"). 

Hence we find from 

e=r-d=rX3o". 

For small angles we can substitute for one second the value • 
1:206265; hence for an interocular distance of 65 mm., 

no 0.065 ^ . 

°-°^^ = ^^6^S' °' ''=^206265; 

r=446.9 m., or in round numbers, 450 m. 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 313 

The "■hunting'" or "sporting^' telemeter has a base of 32 cm., 
a telescopic magnification of 4, and a scale for reading distances 
from 20 to 500 meters. Objects beyond 8000 meters appear as 
infinitely far off. The weight of this instrument is about 2 J kgr. 
Fig. I, Plate CVIII, shows a general view of the Zeiss sporting- 
telemeter. 

The "infantry telemeter" has a 51-cm. base, a telescopic 
magnification of 8, and a scale for distance reading from 90 
to 3000 meters. Objects beyond 28 km. appear at infinite dis- 
tance. This instrument has a weight of about 3 J kg., and it 
cannot well be manipulated without a support. The telemeter 
with suitable (tubular) stand weighs 6J to 9^ kg. 

The so-called "stand telemeters" (the central part of one is 
shown in Fig. 2, Plate CVIII) have a 1.44-m. base, a telescopic 
magnification of 23, and a scale for reading distances ranging 
from 500 to 8000. meters. Objects beyond 230 km. appear at 
infinite distance. 'Jhese stereotelemeters require a rigid sup- 
port, and the Zeiss firm has devised a special tripod for them. 
The weight without tripod is 15^ kg. The packing-case weighs 
20^ kg. The tripod with fork- rest and tilting joint weighs 18^ kg. 

The attachment marked B in Fig. 2, Plate CVIII, is to be 
secured to the right eyepiece for illuminating the image planes 
in the binocular microscope of the stand telemeter when ad- 
justing the ocular scale in the stereoscopic image plane. It is 
used in coimection with a pair of Gautier-Prandl prisms that 
may be adjusted over the objectives, as indicated by dotted lines 
in the diagram, Plate CIX. When these Gautier-Prandl and 
the eyepiece prisms are in the position shown, the light-rays 
entering 0' through the prism B will pass from m' to O', thence 
through the right Gautier-Prandl prism, through the left prism, 
through O and m, emerging through 0, illuminating both image 
planes in their course, as indicated by the dotted line, Plate CIX. 

We will not refer here to the adjustments nor to detailed 
directions for using the difi^erent types of stereotelemeters, as 
printed directions are sent out with every instrument. 



314 PHOTOTOPOGRAPHIC IIETHODS AND INSTRUMENTS. 

The errors affecting the readings of these telemeters increase 
with the square of the distance. There is a certain zone of un- 
certainty for all points in a frontal plane. That is to say, that 
for a certain reading made with the well-adjusted telemeter the 
distance, as read off on the aerial scale, may be too long or too 
short by a certain amount; each reading will be affected by a 
positive or a negative error. 

In the table opposite the probable errors, 



effecting different distances, read with the three types of telem- 
eters, have been tabulated for comparison. 

To use a stereoscopic telemeter successfully the observer 
must be able to see " stereoscopically " ; this, of course, excludes 
all persons with defective vision or who have developed the power 
of vision in one eye at the expense of the other, or whose eyes 
are abnormally spaced, less than 58 or more than 72 mm. apart. 
Both eyes should be used simultaneously, and it will require quite 
a little practice before the observer will become expert in dis- 
tinguishing differences in the distance between objects appar- 
ently close together in the stereoscopic field and yet in different 
frontal planes. 

To test an observer's ability to see stereoscopically. Dr. 
Pulfrich has constructed a stereoscopic " test-plate " (" Prue- 
fungstafei fuer stereoskopisches Sehen "), which is issued together 
with his treatise, " Ueber eine Pruefungstafel fuer stereoskop- 
isches Sehen," published in the Zeitschrift fuer instrumenten- 
kunde. Heft 9, 1901. The figures and diagrams shown in 
that test-plate not only give the means for a quantitative test 
but they are also designed for making a qualitative test of the 
observer's stereoscopic vision. 

Plates CII and CHI show roughly made diagrams for test- 
ing stereoscopic vision in the quantitative sense only. The 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 315 





Probable Error in Meters for the 


Distance = D 










in Meters. 


Sporting Telemeter: 


Infantry Telemeter: 


Stand Telemeter: 






Base — 32 cni., 


Base = si cm., 


Base = i.44 m., 






Magnification 4. 


Magnification S. 


Magnification 33. 




20 


0.05 








3° 


0. II 








40 


0.20 








50 


0.31 




* 




75 


0.70 








90 




0-3 






100 


1-25 


0-3 






125 




°-S 






150 


;..8 


0.8 






175 




1. 1 






200 


s-° 


1.4 






300 


"•3 


3-2 






400 


20.0 


6 






500 


31-3 


9 


2.5 




600 




13 


2.8 




700 




17 


3-6 




75° 






4.4 




800 


Objects beyond 


23 


6.7 




900 


8 km. appear as 


29 


9.9 




1000 


at infinite distance 


35 


13 




1250 




55 


18 




1500 




80 


22 




1750 




109 


27 




2000 




140 


33 




2250 




181 


39 




2500 




223 


54 




2750 




270 


70 




3000 




320 


no 
156 




35°° 








4000 






21S 




5000 




Objects beyond 28 


278 




6000 




km. appear as at 


352 




7000 




infinite distance 


440 




8000 










9000 










lOOOO 












A 




Objects beyond 
km. appear as 
finite 


230 

in- 



circles, Plate CII, are numbered with their increase in distance, 
No. I being nearest and No. 9 farthest. In Plate CIII the pyra- 
mid is nearest the eyes; then follow the cross, concentric rings, 
circle near the cross, large ring or circle (inclosing the figures), 
central wheel with four spokes, cube, smaller circle (below the 



3l6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

cube), base of large cone, and finally the base of the small cone. 
The axes of the two cones are not in line and the base of the 
pyramid is not in a plane parallel with the large circle, its left 
corner being tilted up a little. The Maltese cross, too, has its 
upper two arms tilted forward toward the observer, the end 
of the left upper arm being somewhat nearer than the upper 
end of the right arm. A careful examination of these plates 
will show whether the observer can see stereoscopically. 

In looking into the stereotelemeter the aerial distance scale 
should appear free in space, in a plane slightly raised toward 
the distant end of the scale. As soon as the particular part 
of the scale has been determined, by inspection, which coincides 
with the object, the distance of the latter is obtained within a 
certain margin of limitation, as already referred to in the pre- 
ceding paragraphs. 

The more expert the observer, the smaller the limit of differ- 
entiation will be, although there will always' remain a certain 
margin of uncertainty corresponding to the limit of power of 
the stereoscopic definition, as noted in the table previously 
cited. 

To become efficient in the rapid use of the stereotelemeter, 
it is essential that the observer make himself thoroughly familiar 
with the divisions of the aerial scale of his instrument. It should 
be noted that the subdivisions of- the scale apparently grow 
smaller with increasing distance, and the observer should not 
only be able to read off each actual scale mark quickly and cor- 
rectly, but he should also be trained to estimate fractions of the 
subdivisional scale lengths accurately. (Attention may be called 
to the fact that the nearest half- reading between two successive 
scale marks falls a little beyond the space center, the quarter 
a little beyond the geometrical quarter-space, etc.) 

Plate CI represents the measuring-scale used in the Carl 
Zeiss hunting stereotelemeter. The marks of this scale are 
arranged in four sections, the scale appearing as a zigzag line. 
The first section, from 20 to 25 m., apparently appears in front 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 317 

of the diaphragm or circle which incloses the scale. The four 
sections control distances as follows: 

(i) 20 to 25 m., divided into i-meter spaces; 

(2) 25 to 50 m., divided into i -meter spaces; 

(3) 50 to 70 m., divided into 2, and 70 to 100 m., into 5 m. 

spaces; 

(4) 100 to 160 m., in 10, 160 to 300 m., in 20, and 300 to 

500 m., divided into 50 m. spaces. 

The reproduction of this scale on Plate CI is faulty; inasmuch 
as the triangular division marks 4 and 6, representing the 40 and 
60 m. divisions of the scale, are considerably out of line, some 
of the other marks show similar imperfections, but less marked 
than these two. 

The glass plate having the aerial distance scale etched into its 
surface is also provided with a transverse scale (divided into 
twenty equal parts, Plate CI) for measuring widths and heights 
of objects. In the stereoscopic fields of the telemeters both 
scales stand out very clear and distinct, being photographic 
reductions of the large-scale originals. 

For the first practice work with the stereotelemeter, well- 
defined objects, preferably those that are silhouetted against 
the sky, should be selected. The instrument is first directed 
toward the sky, the interocular distance adjusted, and the eye- 
pieces focused. The instrument is now gently revolved down- 
ward until the object ranged upon appears in the lower field, 
when the aerial scale should appear free in space above the 
object. It will now remain for the obsefver to find that place 
of the scale which coincides with the object (which will be in 
the same frontal plane with it) to estimate the fractional dis- 
tance from nearest scale mark to object. 

The scale divisions are indicated by small triangles, the acute 
angle pointing downward, and to determine the position of an 
object with refeirence to the scale, the highest point of the object 



3l8 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

is brought as close as possible to the imaginary line, connecting 
the scale marks near the object, when the mark to the near side 
of the observer and close to the object is picked out, the observer 
estimating the distance of the object beyond this mark to arrive 
at the actual distance. Should the mark just beyond the object 
be observed, instead the nearer one as just stated, the tendency 
generally seems to be towards overestimating the distance. 

The Carl Zeiss firm has also constructed a stereoscopic telem- 
eter without an aerial scale. The stereoscopic field of this 
telemeter shows an index mark which is movable by the aid of a 
micrometer screw. With, this instrument several independent 
measurements of the distance between two objects may be made, 
similar in manner to the method of repetitions. 

The Carl Zeiss stand stereotelemeter may be used at night 
for estimating or measuring the distances of lights (vessels, light- 
houses, etc.), by illuminating the scale on the diaphragm plate, 
means for doing this being provided if a suitable lantern be at 
hand. 

The binocular microscope of, the stereocomparator, of which 
a description follows, is built after the model of the stereo- 
telemeter, its telescope with prisms being here replaced by a 
binocular microscope with reflecting mirrors, through which the 
upright stereoscopic images are examined under enlargement. 

B. The Stereocomparator and the Stereophotographic 
Surveying Method. 

The angle included at a distant point between the visual ■ 
rays from the eyes of the observer is known as the parallactic 
angle, or parallax. The parallax increases when the point 
approaches the observer and vice versa. For a point at in- 
finite distance the two visual rays will be parallel; the parallax 
will be =o. 

The estimation of the distance of a point, when observed 
with both eyes, depends largely upon the subconscious gaug- 
ing, or mental measurement of the parallax. The distance of 



STEREOSCOPIC TELEMETER AND STEKEOCOMPAKATOE. 319 

an object when viewed with one eye only may still be estimated, 
but in monocular vision such estimates must be based mainly 
upon the degree of diminution in the apparent size of the object 
if a familiar one, or the reduction in size of closely neighboring 
bodies of which the actual sizes are known; for instance, a per- 
son or an animal that may be standing near the distant object. 

The stereophotogrammetric method is based on measure- 
ments made simultaneously on two stereoscopic pictures show- 
ing the same terrene and exposed from aU the ends of a compara- 
tively short base line. These simultaneous measurements, on 
corresponding plate pairs, of the coordinates to locate identical 
terrene points with reference to the horizon and principal lines 
are made with the " stereocomparator," ingeniously devised by 
Dr. C. Pulfrich, a member of the scientific staff of the Carl Zeiss 
Optical Works in Jena. 

The principle imderlying the construction of the stereo- 
comparator may be elucidated in the following manner, sug- 
gested by P. Seliger of the Prussian Topographic Bureau. Two 
pointers, made of black wire of equal thickness and of equal 
lengths, when suspended over the face of two stereoscopic views 
secured in a stereoscope will become superimposed and appear 
as a single wire in the stereoscopic image of the two views. 

If we now move one wire over the face of the picture, bring- 
ing it a little closer to the wire over the other picture (we reduce 
the parallax), the apparent position of the wire index in the 
stereoscopic image will have become more distant, and as soon 
as the distance between the two wires is made to coincide with 
the interocular distance the superimposed images of the wires 
will appear infinitely far off in the stereoscopic field. By thus 
changing the relative positions of the two wires, the observer 
can transfer their stereoscopic image to any point of the stereo- 
scopic field. 

All points of the stereoscopic image that are in a vertical plane 
parallel to the stereoscopic base have the same vertical distance 
from the base line.- Since the vertical distance of such frontal 



. 320 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

plane has the sams ratio to the base length as the focal length 
' of the camera has to the parallax, we can compute such distance 
if we know the parallax, the base and the focal length being 
constant for every stereoscopic picture pair. 

The stereoscopic picture pairs are placed on the stereocom- 
parator to be measured through a binocular microscope similar 
in construction as the Zeiss "stereoscopic telemeter," by means 
of which the two coordinates and the parallax of any pictured 
point may be measured, after optical bisection, by reading the 
corresponding verniers of the three scales that are connected 
with the stereocomparator. The data thus obtained suffice for 
the cartographic location of the point in both the horizontal and 
vertical sense. 

Referring to Plate CIV, which shows the general arrangement 
of the stereocomparator, we designate by 

Pi and P2 the left and the right pictures; 

H the rack-and-pinion motion for moving both pictures together 

from left to right and vice versa; 
M' a screw for moving the right picture alone and in the same 

sense as the motion imparted by iJ; 
M a screw for turning the right picture; 
N a screw for raising or lowering the right picture; 
T the turntable for the right picture; 
S a screw for moving the left picture independently from right 

to left and vice versa; 
R and h plates running in grooves, to be raised, lowered, or 

moved transversely; 
A and B scales for measuring the coordinates of points pictured 

on both plates; 
a scale for measuring the parallax of pictured points; 
C binpcular microscope; it may be raised or lowered by turning 

the screw V, the amount of such change in altitude to be 

read off within o.i mm. on the vernier of scale B, 

To be viewed stereoscopically the two pictures Pi and P2 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 32 1 

are placed on their plates, R and T, in a. position correspond- 
ing to that they had when the exposure was made, with their 
principal lines made parallel and vertical. If the base-line ends 
are not of the same elevation, the right plate is raised or lowered 
until corresponding points appear equally high when examined 
through the binocular microscope. 

The binocular microscope may be moved toward or away 
from the negatives by turning E and each eyepiece is inde- 
pendently adjustable to the eyes of the observer. The objec- 
tives also are movable in the direction of the optical axes, to 
give a range of magnification of 4 to 8 diameters. Two index 
marks have been pro\'ided, one in each image plane of the micro- 
scopes, for bisecting identical terrene points. By turning the 
micrometer screw F the index of the right microscope may 
be moved, changing the apparent distance of its stereoscopic 
image. For one turn of the micrometer F the index will 
be moved 0.2 mm., which would correspond with a change of 
the right plate of 0.3, 0.2, and 0.15 mm., using a magnification 
of 4, 6, and 8, respectively. 

To find the parallax of a pictured point the stereoscopic 
image of the index mark is set at apparent infinite distance and 
both plates are now moved and adjusted until the terrene point 
to be measured coincides with this index mark. The motion 
of the right plate, accomplished by turning the screw M', 
to bring the mark and point into contact, is read off on the vernier 
a, which reads to 0.02 mm. By estimation, however, the 
value for the parallax may be obtained within o.oi mm. After 
the index has been made to bisect the point the readings of 
the verniers A and B wiU give the coordinates of the pictured 
point with reference to the left picture (left base station). 

The main advantage claimed for the use of the stereocom- 
parator in phototopography rests in the fact that one pointing 
of the index on the point at once gives the elements for the car- 
tographic location of the point, whereas with the plane-table or 
tadial method three distinct measurements would have to be 



322 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

made before the pictured point may be plotted, involving the 
separate measures of two abscissae and one ordinate, Witk 
the stereocomparator the coordinates are measured directly on 
the negatives with microscopes and verniers, and the accuracy 
obtained should be greater than obtainable with the plane-table 
or radial method, using paper prints, dividers, and scales. 

The index mark being placed upon each terrene point that 
is to be plotted from the pairs of pictures, it is evident that the 
better the definition of such points the closer will be their sub- 
sequent cartographic location. 

The three readings made on the scales A, B, and a give the 
data for locating any pictured point (bisected with the movable 
index mark) in regard to direction, distance, and elevation, and 
with reference to the left station. 

The abscissa x (Plate CV, Fig. i), read off on scale A, is 
plotted in the usual manner by laying off the distance on the 
picture trace T from the principal point O, a line SR, drawn 
through the end of x from the left station S gives the line of 
direction to the bisected point P. 

The distance is ascertained from the vernier reading of scale 
a which gives the linear parallax of the bisected point. The 
distance may be computed from the equation 

A:B=l:a, 

a 

where B is the distance between the two stations, 

/ the constant focal length of the camera, and 
a the parallax as read off on the scale. 

In Fig. I, Plate CV, TT represents the picture trace, 
X the abscissa of the point P, the plotted position of which 
will be on the radial SR. The value for A, as computed 
above, is laid off on the principal line from S. A parallel 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 323 

to TT, drawn through the end of A, will bisect SR in P, 
■which is the horizontal projection of the bisected point. 

Dr. Pulfrich recommends a graphiced solution of the equation 

a 

of which the product B-f is a, constant for every pair of pictures. 
Referring to Fig. 2, Plate CV, 

SSi = B = hase line; 

rT'= picture trace of left picture; 

a= vernier reading for parallax, laid off on TT from O. 

If we now draw the radial Sr, the intersection of the latter 
with Sib, drawn parallel with SO, ^^•ill cut off the distance 
A on Sib, and the point to be plotted will be on the line 
MN, drawn parallel to TT at the distance A from S. The 
plotted position may- now readily be found by laying off the 
abscissa x from O and drawing the radial SR; the intersection 
of the latter with MN locates the plotted position of the point P. 

Fig. 3, Plate CV, shows the simple device suggested by Dr. 
Pulfrich for the graphical solution of the equation for A. 
A drawing-board is covered with a tough paper and a line SO 
is drawn parallel with its lower edge. On SO, at a distance 
1.5 /. from S, a vertical UT is erected and a scale of divisional 
parts equal to 1.5 mm. is laid off on UT. The line SO 
is provided with a 1000-meter graduation in the plotting-scale 
(say 1:25000). LL is a straight edge secured to the board 
parallel with SO. G is a transparent film of celluloid 
attached to a brass strip k in such a way that it may readUy 
be slid along LL over SO. This transparent plate G has two 
^aduation, tt and f, in the reduced scale of the map. The 
ruler SR, also provided with the reduced scale of the map 
(i : 25000), may be revolved about the pin in 5. 

To use this device the base line for a pair of plates is laid 



324 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

off on tt, say in tenfold scale of the map. After the parallax 
a has been read off on the scale of the stereocomparator, 
the ruler SR is placed on UT to cut off the length loa, and G is 
moved until the base end, marked off on //, coincides with the 
fiducial edge of SR. The corresponding value for A may now 
be read off on SO, using the scale t' for the subdivisional parts 
of SO. 

To find the distance SP of the plotted point from the 
left station S, the position of G is maintained unchanged, 
while the edge of SR is made to coincide with that division 
mark of the scale UT which corresponds with the abscissa 
X of the pictured point. The distance SP may now be read off 
on SR to be transferred to the radial SR, Fig. i, Plate CV. 

The difference in elevation of pictured -point and left-base 
station, 

may also be found graphically. After the distance SP has 
been read off, G is held in the same position and SR is 
brought to coincide with that scale division on UT which 
corresponds with the ordinate (read off on scale B of the 
stereocomparator), when the reading of the scale //, between 
SO and SR, will give the value for h in meters. Instead 
of moving SR to bisect the end of the ordinate y on UT 
it is desirable to use a multiple part of y, say loy, and 
divide the final reading by lo. The elevation of the plotted 
point is now derived from the elevation of the left station in the 
customary manner, referring h to the elevation of the horizon 
line of the instrument at the station and allowing for curvature 
and refraction for points over 2000 m. distant from S. 

The apparent length of the index mark in the stereoscopic 
image plane of course corresponds to different heights, accord- 
ing to the distance of the bisected object; the relation between 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 325 

both, however, may readily be ascertained when the actual or 
absolute length of the index mark be known. This length is best 
found by bringing the upper end of the mark into contact with 
a well-defined horizontal line in the picture and noting the read- 
ing of the scale of ordinates (scale B), then bringing the lower 
end of the mark into contact with the same horizontal line and 
again noting the vernier reading of the scale B. The differ- 
ence between the two readings will equal the absolute length 
of the index mark (=/»)• 

The comparative length value = J/ of the index mark for a 
distance=-4 may be computed from the formula 

If .the value for m be found 0.75 mm. and the constant 
focal length of the camera be 250 mm. the value for J/ would 
be 0.003 -4. Xow, say the lower end of the index mark coin- 
cides ^-ith the base of an embankment at an apparent distance 
from 5=j1 = 4ooo m., while the crown of the embankment may 
bisect the iudex line at one third of its length, the absolute 
height of the embankment would then be 

4000X0.003 

=4irL 

3 

It is ver}- essential that each pair of plates be exposed in a 
vertical plane containing the base line or being parallel with it. 
If such be the case, points lying at infini te distance in the ver- 
tical planes of the objectives should appear pictured in the prin- 
cipal lines of the plates. If either of the plates, say P, includes 
an angle= o with the vertical plane of the other, the distant 
point wiU be pictured to one side of the principal line (see Fig. i, 
Plate C\T). The distant point .4 will be pictured at a 1 in 
plate pii, but the principal point wiU be to one side, at an. 



326 



PHOTOtOPOGRAPEIC METHODS AND INSTRUMENTS. 



The plates after being placed on the holders ot the stereocom- 
parator are adjusted by means of the horizon and principal 
lines, and in this case all parallax values will be measured too 
small by 

A correct measurement of the length of the base in rough 
mountain regions often offers serious difficulties, telemeter read- 
ings generally being the only available means for measuring these 
base lines. Any error made in the base will affect all distances 
determined from its left station, and such being the case it would 
appear advisable to select relatively long base lines. The lengths 
of the latter, however, are controlled by the fact that picture 
pairs can no longer be viewed stereoscopically in their full extent 
when the length of the base exceeds a certain limit. Pictures 
obtained from the ends of too long a base will have but limited 
distance zones that may be examined stereoscopically through the 
binocular microscopes; areas outside of these, both near and far, 
will appear blurred and indistinct. The examination of such 
plates through the microscopes is not only very trying to the 
eyes, but the observer also loses the general view of the terrene 
and he will have to refocus the microscopes for every change in 
distance. 

For a constant focal length of 241.5 mm. and an error in parallax 
of o.oi mm., errors in distances may be made, for base lengths 
of 50, 100, 200, and 300 meters, as hsted in the following table: 



Distance of 




Length of Base Line in Meters. 
















in Meters. 














50 m. 


100 m. 


200 m. 


300 m. 




1000 


0.8 


0.4 


0.2 


O.I 




2000 


3 


3 


1-7 


0.8 


0-5 




3000 


7 


4 


3-7 


1.9 


1.2 




4000 


13 


2 


6.6 


3-3 


2.2 




5000 


20 


7 


10.3 


S-2 


3-4 




7500 


46 


8 


23-4 


II. 7 


7-7 




lOOOO 


82 


8 


41.4 


20.7 


13-8 





STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR- 327 

If errors in position of ±15 m. be permissible in rough moun- 
tain work plotted in 1:25000 scale, a mean error of ±3 m. may 
be accepted for the same kind of work plotted in i : loooo scale. 

For a parallax error not exceeding ±0.01 mm. distances to 
6000 meters from the base stations should be controlled, and 
for the 1 125000 plotting-scale base lines of 100 meters preferably 
should be selected. For the i : loooo scale a loo-meter base 
should be selected for distances up to 3000 meters and a 200- 
meter base for 4000 meters, etc. If the objective has a focal 
length shorter than 240 mm. the base should be made propor- 
tionately longer. For instance, for a focal length of 180 mm. 
the base lines as given above should be increased by one quarter. 

The terrene pictured on a pair of stereoscopic plates, when 
examined through the binocular microscopes, appears very much 
like a relief model of the coimtry, the changes in the surface for- 
mation being far more clearly shown than in the landscape itself 
when viewed from either of the two base stations. 

For the best iconometric results each plate should contain 
from 6 to 12 control points of known elevations and positions 
(tertiary triangulation points). After the left base station and 
all the control points have been plotted, the two stereoscopic 
plates are placed on the comparator frame to be adjusted in 
the manner already described. After the parallaxes, abscissae, 
and ordinates of all the control points that are pictured on the 
plates have been measured and tabulated, the picture trace of 
the left picture is plotted, based on the computation of the radials 
drawn to two control points. It is preferable to select two extra 
axial points, one near the left and one near the right margin of 
the plate, for plotting the picture trace. The position of the 
latter is checked by means of the abscissae of the other pictured 
control points. It would not be sufficientiy accurate for our 
purpose (" stereophotogrammetry ") to plot the pictiure trace 
hy means of a paper strip, as generally used in the plane-table 
or intersection method. 

Parallel with the picture trace and from ij to 2 times its 



328 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

distance from the station point a scale is drawn having the 
same graduation and numbering as the scale (of abscissa) A 
of the comparator, the divisional parts of course being enlarged, 
according to the selected distance, i^ to 2 times. A ruler having 
the plotting-scale along its fiducial edge may be secured to the 
station point in such manner as to revolve about the station 
with the zero mark of the scale as pivot. With these means 
the pictured points may be quickly plotted without actually 
drawing their lines of direction, which radiate from the station. 

The next step is to check the position of the horizon line by 
means of the ordinates of the pictured control points. Any cor- 
rection affecting all points alike is made by changing the zero 
mark of the scale B on the comparator. If the horizon line 
has to be raised or lowered on one side, the plate will have to 
be turned correspondingly on the holder of the comparator. In 
the latter case the adjustments of both plates on the comparator 
will have to be repeated to allow for the change just made. 

With the measured parallax values a the distances A 
of the control points are computed and compared with those 
of the plotted points. Discrepancies A between these exceed- 
ing the amount due to errors in parallax of ±0.01 mm. would 
point toward an error in " swing " (J«) during the exposure 
of the plate, errors in base measure (i^), or toward errors 
due to both. We may, therefore, express these discrepancies 
by the equation 

If the error in base measure equals b and the error in 
parallax, due to the "swing of plate," d, equals s=ld, we 
will have the equations 

A_ 

B 



^,= ^-h 



A ^' 



STEKEOSCOPIC TELEMETER AND STEREOCOMP ABATOR. 329 

We can now compute the errors A and A\ (discrepancies 
between the computed and plotted distances A) from the 
parallaxes of two pictured control points and substitute these 
values for A and ii in the equations 

^'=B^^-B:f^ 



after which the values for the base-line error (6) and the 
error in paraUax (s) may be computed and applied to the 
base-line and paraUax values. 

A better way would be to use all the control points, tabulate 
the errors (J) graphically, and find the values for Jj and Jg by 
interpolation, as shown on Plate CVII. 

The abscissae of the control points are plotted in their true 
lengths and the corresponding errors A are plotted as ordinates, 
giving the points 47, 48, 44, 28, 38, 32, 34. A curve passing 
through the initial point O is laid through this series of points. 
To separate the ordinates, J, of this curve, OC, into the 
component parts, A^ and A^, a tangent OG through O to the 
curve OC is to be drawn in such manner that the upper sec- 
tions m, n, 0, etc., increase in length with the squares of A 
{o=/\m, p=4n, q=40, etc.), as the increase in the errors Ag is 
directly proportional to the squares of the distances A and 
the errors Aj, increase in the same ratio as the distances A. 

With a pair of dividers and a ruler the position of OG 

may be located tentatively. For an error in the base line, b = 0, 

Bf 
the curve OC will be a parabola, havmg the parameter = ; 

for an error in swing, 5= O, a straight line will replace the curve, 

h 
and A = A •-=-. 



330 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The lower ordinate section serves for the determination of 
the base-line correction, b = -j-^^, and the upper section gives 

Bj 
the correction for the parallaxes ■y=~72"^<f 

The general course of the curve OC will be a criterion 
of the errors affecting a pair of plates, showing whether they are 
due to regular causes or whether errors of level adjustments, 
errors in computation, etc., have crept in also. If no smooth 
compensating curve may be drawn to harmonize with the series 
of plotted points, errors outside of those referred to in the pre- 
ceding paragraphs should be looked for. 

A serious error in the swing of the plate may affect the curve 
in a marked manner. The correction applied to the parallax, 
as referred to in the preceding, neutralizes only the constant 
8f. It corrects the position of the principal hne with refer- 
ence to the pictured points, but when there is a decided swing 
in the plate the parallax, for points to either side of the principal 
line, will be in error, even after the correction 5 for the paral- 
laxes has been applied. 

After the correction df has been applied, the plates may 
yet have the relative positions indicated in Fig. i, Plate CVI, 
where points at infinite distance and situated in the principal 
plane will be pictured in the principal lines of both plates, whereas 
the images a of a distant point A, lying to one side of the 
principal plane, will be pictured at a and an, instead of at a 
and fli. In lieu of the correct parallax (xi—x) we obtain 
the smaller value (xn—x), referring to Fig. 2, Plate CVI. The 

... X'^ 

error thus remainmg, which may be expressed siS J = -rd, increases 

rapidly with an increase in the length of abscissa; it is positive 
on one side of the principal line and negative on the other, being 
±0 for points on the principal line. It is a prime requisite, 
therefore, to expose pairs of plates as near as possible in a ver- 
tical plane parallel with the base. 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 33 1 

After a plate pair has been tested and after the corrections 
found necessary have been applied the iconometric mensuration 
may be commenced. The pictured points may be plotted by 
means of their lines of direction, based on the abscissa values, 
recorded on the scale A, and its horizontal distance from the 
left station, based on the measured parallax as given on scale a 
of the stereocomparator. The difference in elevation between 
the station and the plotted point may be computed from the 
reading of scale B. To ascertain the parallaxes of the pictured 
points the index mark is moVed from point to point in the stereo- 
scopic image field, very much in the same way as the telemeter 
is caried from point to point in the field when reading distances.. 
It is evident that the index mark may readily be moved to bisect 
points in the image field that would be inaccessible for the ordinary 
telemeter in the field. The distances, obtainable by moving 
the index mark in the stereoscopic field, considerably exceed 
those measured with the telemeter and the time required for 
obtaining these distances stereoscopically is so short that the 
advantages of the stereoscopic method over the plane-table and 
tachymetric methods are out of question for topographic recon- 
naissance work in rough mountains. 

The stereocomparator, furthermore, is peculiarly well fitted 
for a quick location of points having the same elevation; the 
index mark may be used to trace out the contours in the stereo- 
scopic field. Points may also be readily located that are in 
the same frontal plane, in the same plane parallel with the base 
line. Actual profiles parallel with the picture traces may thus 
be. run out and by locating points to either side of the profile, 
using the micrometer screw of the binocular microscope for this 
purpose, terrene strips of 150 to 250 meters width (scale i : 25000) 
may be developed, which will form the base for the subsequent 
orographic development of the topography. 

The positions of points that have been plotted by the usual 
method of direction and distance may be checked by referring 
them to the plotted positions of near-by pictured control points. 



332 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

The stereoscopic photogrammetric methods evidently offer a 
wide field for application to ascertain changes that may have 
occurred during periods of time that are allowed to elapse before 
taking a new set of pictures from the same base line (or at least 
from the same vicinity). The examination of two stereoscopic 
plate pairs of a glacier, for instance, would at once show any 
change in form or location if the left plate of one pair be examined 
in the stereocomparator with the right plate of the second pair, 
both pairs being obtained at different times from the same base 
line. 

Good results may be expected from this method, if applied 
by the navy, for mapping coast lines without making a landing, 
by taking simultaneous views of the coast, fortifications, etc., 
from a vessel (a base line being measured on the deck between 
the camera stations), noting the position of the vessel on the 
chart at the time of exposure. 

The use of the stereocomparator may also be recommended 
for recording the positions of moving bodies (army corps, fleets 
during maneuvers or in time of war), making plans of inaccessible 
objects, for the mapping of cities, for making profiles, plans, and 
relief models of areas to be studied for comparative locations of 
roads, railroads, irrigation plants, etc. 

To recapitulate, the actual mapping of the terrene details^ 
based on the examination of stereoscopic picture pairs, may be 
made: 

(i) With relation to a series of control points plotted from 
data obtained directly with the stereocomparator; 

(2) With relation to a series of contours obtained directly 

from the picture pairs, or 

(3) By means of profiles composed of points having the same 

parallax, i.e., points in frontal planes. 

Of the many-fold uses to which the stereocomparator is adapted 

we may mention stellar surveys, the testing of banknotes, the 

comparison of scales and their prototypes, comparing facsimiles 

and replica of various kinds, the study of animals in motion^ 



STEREOSCOPIC TELEMETER AND STEREOCOMPARATOR. 333 

migratory-bird flights, changes in the northern lights, cloud ele- 
vations, terrene changes due to landsUdes, volcanic eruptions, 
inundations, forest fires, etc., for the study of effects produced 
by bombardments and explosives, changes in sand dunes, etc. 

In the preceding paragraphs it was assumed that each pair 
of plates was not only exposed in the same plane (the plane con- 
taining the base line or laid parallel with it), but this plane was • 
also supposed to be vertical. The verticahty, of course, greatly 
facilitates and simplifies the iconometric constructions, yet it is 
not a sine qua non. If a plate pair be exposed in the same inclined 
plane, all that has been said about the stereotopographic method 
still holds good if the angle of inclination of the plane containing 
both plates during their exposures be measured and taken into 
accoimt. 

If the landscape pictures on the inclined plates could be trans- 
ferred to vertical plates by photography, the latter could be used 
on the stereocomparator just as if the plate pair had been exposed 
in the vertical plane originally. 

The inclined-plate position will often be xmavoidable in making 
stereophototopographic surveys from the decks of vessels, and 
even in mountain work a suitable location for the base line will 
sometimes necessitate exposures to be made on inclined plates 
in order to control deep valleys or high elevations from the two 
base stations. 



CHAPTER XI. 
PHOTOGRAPHIC OPERATIONS IN THE FIELD. 

Until now the principal operations have been considered 
for obtaining the so-called " latent " or invisible image on the 
exposed plate, which is still to be converted into the " negative," 
in which form the terrene image is used, either directly or indi- 
rectly, for the iconometric plotting of the pictured topographic 
features. 

The work of developing and fixing the negatives of an exten- 
sive photographic survey is best done by a photographic expert 
who has made special studies and experiments for this purpose. 
He should be thoroughly famiUar with the laws that control the 
changes, both chemical and physical, which take place in the 
compositions of the sensitized coatings of the photographic plates, 
when they are exposed to the action of light, as well as those 
which control the changes in the chemical compositions of the 
sensitized films when the plates are immersed in the developing, 
toning, and fixing baths. 

Still, every phototopographer should be sufficiently familiar 
with the general routine practice of photography to develop 
some " trial " or " test " plates understand ingly and successfully 
while he is yet in the field. 

At least a few plates taken at random from every batch 
originally packed together and which are hkely to have passed 
through the same conditions during transportation should be 
developed, while still in the locality where the exposures were 
made, to feel satisfied that no plates were spoiled and also to 
feel reasonably assured that the exposures were correctly timed. 

334 



EXPOSURE OF A PHOTOGRAPHIC DRY-PLATE. 335 

The wisdom of developing test-plates, to avoid loss of valuable 
time and material by incorrect exposures or by the use of spoiled 
plates, is beyond dispute. If all development of plates be post- 
poned until after the return of the expedition, defective plates 
cannot be replaced without expending large sums of money, and 
the results of the expedition may be robbed of much, if not of 
all, practical value. 

Whenever there is danger of losing undeveloped plates through 
careless and ruthless inspection of baggage on frontiers, or through 
the inquisitiveness of packers, to whom the transportation of the 
plates must be intrusted, it is of course advisable to develop all 
the plates of the survey in the field, pari passu with the progress 
of the survey. 

The principal records of the season's work, regarding the 
topography at least, consist in a series of undeveloped plates,, 
and the phototopographer should feel reasonably certain that 
they are of as good a quality as could be obtained under the con- 
ditions of climate and surroundings prevailing at the time of 
their exposures. 

We will give in the following a cursory review of those opera- 
tions to which an exposed plate is to be subjected before it is 
converted into the permanent negative, and with which the 
phototopographer should be familiar to enable him, for the rea- 
sons just stated, to develop some test-plates while he is still 
in the locality where the exposures were made. 

I. General Remarks on the Exposure of a Photographic 

Dry-plate. 

When the sensitized coating of a photographic plate is exposed 
to the action of the rays of so-called white light — solar light — 
certain effects upon the chemical composition of the coating 
will be produced, consisting primarily in a reduction of the silver 
haloids that are embodied in the gelatine coating of the dry-plate 
into an imstable condition, permitting a deposit of metallic (black) 



336 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

silver to be readily made upon the plate when the latter is immersed 
in the so-called " developer " (reducing bath), which converts 
the light-sensitive " latent image " of the exposed plate into the 
"negative " of a permanent and stable character. 

The greater the intensity of the light that reaches the plate 
in the camera, or the longer the exposure of the plate to the action 
of the light-rays, the greater will be the amount of reduced silver. 

The quantitative effect, in a given time period, of white 
light upon the sensitized coating of a photographic plate may 
differ perceptibly from the quantitative effect of chromatic or 
color rays, although their qualitative effect upon the silver haloid 
(bromide of silver) is essentially the same. For short exposures 
the quantity of reduced silver may be regarded as directly pro- 
portional to the duration of the exposure. The " density " of a 
negative is more or less great according to the larger or smaller 
amount of reduced silver that has been deposited; density increases 
directly with the length of exposure. 

Photographic dry-plates differ materially regarding their 
" speed," or their sensitiveness to light action. The speed is 
generally indicated by the so-called " sensitometer number," 
ascribed to each emulsion. The same density for two different 
plates, when photographing the same object under identical con- 
ditions, may be attained by giving each plate a different length 
of exposure, corresponding to its sensitometer number, the less 
sensitive plate, of course, being given the longer exposure. 

Under ordinary conditions, three different stages of exposure 
may be considered in practical photography: 

1. Underexposure; 

2. Correct exposure; 

3. Overexposure. 

A fourth stage, the so-called " period of reversal," may pos- 
sibly be reached, but this requires so lengthy an exposure that 
it will rarely be attained, inadvertently, when exposing plates 
for phototopographic purposes. 

An underexposed plate may be recognized by the marked 



EXPOSURE OF A PHOTOGRAPHIC DRY-PLATE. 337 

contrast in the negative between the lights and shadows and a 
general deficiency in details. Such plates will be of httle or no 
value for iconometric plotting. 

An overexposed plate shows Uttle contrast between the lights 
and shadows and the general details will be weak and flat. 

When a plate had been exposed correctly, its scale of grada- 
tion in tint, after proper development, will embrace the widest 
range possible, from pure transparency (white) to black. A 
negative appears transparent where the photographed object 
was dark and vice versa. The negative should be a true inverse 
of the original regarding the light gradations. 

The source of the light-rays which are emanated by any 
object in nature may be a threefold one, comprising: 

1. Rays of direct sunlight; 

2. The less intense rays of diffused skylight; 

3. Rays originating from the foregoing two sources, but 

reaching the subject indirectly after having been re- 
flected from surrounding objects. 

The intensity of the light-rays, generally siunmarized as 
daylight, is subject to many variations. The sunlight alone will 
have a variable intensity at different altitudes and imder differ- 
ent atmospheric conditions, irrespective of the geographic latitude. 

The tendency of aerial perspective is in the direction of diffu- 
sion of sharp outlines of distant objects and toward obHteration 
of details. The skyline of distant mountains becomes merged 
into the so-caUed " blue haze." The nearer sea-level the observer 
is stationed the more indistinct will distant objects become, 
while in high altitudes, with a relatively dry atmosphere, objects 
will be discernible, as to form and color, at far greater distances^ 

Some of the polychromic rays of sunlight, on their passage^ 
through the atmosphere, intervening between the observer and 
the object, will become diffused or absorbed, while others will 
transverse the same without suffering any perceptible modi- 
fications. Color rays near the violet end of the solar spectrmn, 
rays of short wave-lengths, are more largely absorbed by the 



338 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

atmosphere than those of longer wave-lengths near the red 
end of the spectrum. 

We had seen (Chapter VII) that the component rays of so- 
called white light after transmission through a lens will be differ- 
ently refracted and the actinic effects of such refracted rays 
upon the sensitized films will differ according to their colors 
or wave-lengths. Those of short wave-lengths, the ultra violet 
to blue, between the Fraunhofer lines Hi and F, have by far a 
more pronounced chemical action upon the silver haloids of the 
plate coating than light-rays with longer wave-lengths, the green, 
yellow, orange, and red rays, between and beyond the Fraun- 
hofer lines E and A. 

The luminous (optical) effects of the component colors of a 
landscape upon the eye are not identical with the actinic (chem- 
ical) effects upon the photographic plate. The optical effect is 
governed by the various degrees of tint, hue, or shade that the 
several parts of the landscape convey to the eye, some parts 
appearing as light, others in half-light, still others in middle 
tint, half dark, and dark. 

Practical experience, on the other hand, teaches that' the 
various blues and dark greens of the chromatic scale appear darker 
to the eye than the yellows, the reds, and the lighter shades of 
green, yet, when a photographic plate is exposed in the camera 
to both, the combined action of the luminous and actinic light- 
rays of an illuminated landscape, the actinic action of the blue 
rays will be more intense on the sensitized film than that of the 
light-green, the yellow, orange, and red rays. In order to obtain, 
therefore, a clear and well-defined picture of the violet and blue- 
colored parts, the exposure will have to be stopped long before 
the parts having shades of light green, yellow, orange, and red 
have been reproduced on the negative. The resulting mono- 
chrome picture will have a scale of but three-tone gradations — 
light, half-light, and dark — instead of the scale of five gradations 
of tone mentioned abdve. 

Thus it may happen, when an ordinary dry-plate is exposed 



ORTHOCHROMATIC DRY-PLATES AND RAY-FILTERS. 339 

in the camera, the sky and blue- tinted parts in general will be 
overexposed if the exposure had been timed correctly for the 
green, yellow, orange, and red-tinted parts. For the use of the 
phototopographer who desires negatives showing also the dis- 
tant details of the landscape clearly and well defined the ordinary 
dry-plate is inadequate. 

II. Orthochromatic Dry-plates and Ray-filters. 

In the preparation of the sensitive film of the " isochromatic ' 
(rendering all color values evenly well), or " orthochromatic " 
plates (rendering the color values correctly), it has been the 
aim to make them equally sensitive to the actinic action of all 
color-rays, so that during a properly timed exposure all colors 
of the subject may be represented upon the finished negative 
equally correct regarding their respective tints and light values. 
As yet, attempits in this direction have been only partially suc- 
cessful, however. Orthochromatic plates are indeed made 
more sensitive to the actinic effects of red, yellow, and light-green 
rays, but the blue rays remain, even with these plates, consider- 
ably more active than the reds and yellows, and to retard their 
chemical action still more, a so-called " color-screen," or " ray- 
filter," is intei-posed between the plate and the subject. A suit- 
able combination of orthochromatic plate and color-screen makes 
it possible to reproduce landscapes (and colored objects) in 
better harmony regarding chromatic values, reducing the actinic 
power of the rays of long wave-lengths and increasing it for the 
rays of the less refractive end of the solar spectrum. 

At present the orthochromatic plates are prepared by impart- 
ing color . sensitiveness to the gelatino-bromide-silver emulsion 
of the ordinary dry-plates by the addition of certain color ingre- 
dients or " optical sensitizers," like erythrosine, cyanine, rhoda- 
mine, eosine, etc. The nearer these optical sensitizers approach 
a blue shade of polor, the more sensitive the plate will become 
for light-rays of the less refractive end of the solar spectrum. 



340 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS, 

The addition of erythrosine is said to increase the sensitizing 
action of the emulsion for light reds, while rhodamine increases^ 
the same for light greens, extending well toward the yellow and 
light orange. Tetrachlor-tetraethyl-rhodamine-chlorhydrate im- 
parts a more powerful sensitizing action for the orange yellow, 
and green tints. Cyanine has a greater orange sensitiveness than 
either of the ingredients named, excepting, perhaps, the last- 
mentioned dye, but the others have the advantage of not materi- 
ally reducing the general speed or sensitiveness of the plate. 
Valuable experiments in this direction establishing the fore- 
going facts have been made by Dr. Eder, Valenta, Mallmann,. 
Scolik, Schumann, Obemetter, and others, who have published 
formulas for the various optical sensitizers that they recommend 
individually. 

A. Color-screens, or Ray- filters. 

The general introduction of color-sensitive plates has been 
somewhat retarded on account of the necessity of a materially 
increased length of exposure when using a color-screen, pre- 
cluding the use of this combination for all instantaneous work. 

For phototopographic purposes ray-filters are used from a 
bright-yellow to a deep-orange tint, varying with the character 
of the plates and lenses used. In the Canadian surveys, for 
instance, an orange- colored filter was used with the Zeiss Anas- 
tigmat Lens No. 3, Series V, together with Edwards' Iso- 
chromatic Medium Plate, while a light-yellow screen (Car- 
butt's) gave good results in connection with Dallmeyer's W. A. 
Lens and Carbutt's Orthochromatic Plates (sensitometer No. 23), 
for the topographic reconnaissance, made by the U. S. Coast 
and Geodetic Survey, in S.E. Alaska. 

Carbutt's (pale-yellow) screens are composed of two thin 
planorparallel crystal-glass plates cemented together with, bal- 
sam and having the color matter between the plates. They are 
2^ or .3^ inches square and can be placed in grooved pieces of 
wood suitably attached to the back. of the lens board. The 



ORTHOCHEOMATIC DRY-PLATES AND RAY-FILTERS. 341 

screen should always be in position when focusing; when not 
in use it should be kept in a box protected against light. With 
Carbutt's light color-screen (yellow) the action of the chromatic 
rays begins between the Fraunhofer lines C and D and it ends 
between the lines E and F. 

DaUmeyer's yellow screens are fitted into metal settings 
which may be attached to the lens mount, close to one side of 
the diaphragm when a lens doublet is used. 

The Bausch and Lomb filter is in the form of a hollow glass 
cylinder that may be filled with variously colored liquids, to suit 
different optical demands. The piano-parallel ends of these 
cylinders are made of optically worked glass and the whole is 
incased in a metal ring that fits over the lens mount. 

Theoretically, the color-shade of the screen should decrease 
in intensity, from the center toward the edge, in the same ratio 
as the intensity of the illumination of the plate in the camera 
decreases from the center toward the margin and its form should 
be spherical, its center of curvature being in the second nodal 
point of the camera-lens. For all practical purposes, however, 
distortion, due to the use of a piano-parallel screen, placed at 
right angles to the optical axis, is imperceptible, particularly 
when the screen is placed in the nodal plane of the lens and when 
using a relatively small stop. 

Terrene points in the shadows of a landscape receive but 
partial illumination from the sky and atmosphere and only reflected 
light from the surfaces of surrounding bodies; the rays repro- 
ducirig such shadows on the plate will principally belong to 
the violet end of the spectrum. Hence negatives, obtained 
behind yellow color-screens give the shadows in exaggerated 
intensity, particularly when photographing mountain views of 
an Alpine character j since the rarefied air in high altitudes absorbs 
less light than the air in lower altitudes. The exaggerated con- 
trast between the high lights and shadows in such views makes 
it desirable to employ specially prepared plates of a thick emul- 
sion coating, which have the further advantage to widen the 



342 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

range of correct exposure. Thinly coated plates require accu- 
rately timed exposures to avoid a characteristic flatness in their 
negatives. 

B. Halation. 

The naturally sharp outlines defining dark sections in Alpine 
views, or objects with marked contrasts, frequently appear blurred 
and undefined in negatives obtained after a rather lengthy expos- 
ure, such condition being caused by a reflex action of rays that 
have deeply penetrated the emulsion and have been reflected 
from the glass surface immediately below the plate coating. 
This effect, known as " halation of the plate," may be greatly 
reduced by covering the rear surface of the plate with an opaque 
coating of the same refractive index as that of the glass used for 
the plate. Plates provided with such protective backings are 
called " non-halation " or " anti-halation " plates. The back- 
ing of non-halation plates should be removed before these plates 
are subjected to the developing process. 

Besides the increase in range of the gradation of tints in a 
monochrome reproduction of a landscape or multi-colored object, 
color-screens also materially aid in the prevention of halation 
when the necessity arises of having to expose a panorama plate 
directly toward the sun. 

To prevent possible side reflection and permit only such 
rays to reach the plate which are conducive to the production 
of the image, it is recommended to insert one or more diaphragms 
in the camera-box and to paint these, like all other interior 
surfaces of the camera-box, a dull black. Rays reflected by 
the lens surfaces should likewise be excluded from the interior 
of the camera. This is effected, in a measure, in the Zeiss Anastig- 
mat Lens by giving the surface of the back lens a strong curva- 
ture. For similar reasons certain surveying-cameras (Deville's 
and that of the U. S. Coast and Geodetic Survey) are provided 
with a " hood " or " lens shade." 

Plates exposed in the field, when well protected against heat 



COMPARATIVE LIGHT VALUES AND EXPOSURES. 343 

dampness, injurious gases, and, of course, against light, both 
before and after exposure, will preserve the undeveloped image 
in the latent stage almost indefinitely. 



m. Comparative Light Values and Ezposures. 

To secure as much detail in the shadows as possible, the 
plate should be given as lengthy an exposure as it will bear with- 
out becoming overexposed- This length depends upon a series 
of circumstances and conditions; the more important ones are; 

1. The intensity of the Ught that reaches the plate; 

2. The sensitiveness of the plate; 

3. The speed of the lens; 

4. The size of stop used; 

5. The color and illumination of the object; 

6. The character of the color-screen; 

7. The distance of the object from the second nodal plane 

of the lens. 

Success in obtaining clear and well-defined negatives depends 
largely upon properly timed exposures, demanding care, judg- 
ment, and much experience, if the results are to be imiformly 
successful. Various tables (and diagrams) of comparative light 
values and comparative exposures have been computed from 
which much information may be gained simply by inspection, 
to obtain which without such aid would require much experi- 
ence, time, and trouble. Such tables and diagrams, of course, 
vary with the latitude of the place and its altitude above sea- 
level; both, however, may be neglected for exposures made while 
the sun is relatively high, say not below 45°. Under this proviso 
we would have to consider only the conditions of the atmosphere, 
including the illumination, the hour of day, the season of the 
year, the rapidity of the plate, the character of the screen, and 
the lens stop, to ascertain the time for correct exposure. 

Any one who has experimented with a certain brand of plate 
under certain atmospheric conditions, in a known latitude, and 



344, 



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



at a certain elevation above sea-level to ascertain the time required 
for the correct exposure for a certain subject with a certain lens, 
and diaphragm, taking recourse to a table of comparative light 
values, can readily decide wha,t time should be given a similar 
plate under the same conditions, with the same lens and dia- 
phragm, at any other hour of the day and on any other day of 
the year. 

For a correct exposure the time given the light for action 
upon the film should be inversely proportional to the intensity 
of the light emanated from the subject. A subject, for instance, 
requiring six seconds' exposure when the intensity pf the light 
be one, would, under identical conditions, require an exposure 
of two seconds for a light value of three. 

The following table contains the comparative exposures for 
different lens stops and for " open " and " dark " landscapes. 





Comparative Exposures for Different Lens Stops. 


Number of Lens Stop 


2 

F/5 


4 
F/8 


8 
F/ii 


16 

P/16 


F/22 


64 

F/32 


128 

FMs 


256 

F/64. 


Open landscape (seconds). . . . 

Average landscape with fore 
and background of average 
color (seconds) 


1/32 
1/6 


1/16 
1/3 


1/8 
3/4 


1/4 


1/2 
- 3 


I 
6 


2 
12 


4 

24 





The upper line of this table gives the so-called " Uniform 
System " numbers of the lens stops or diaphragm apertures, 
which numbers have the same ratio to each other as the areas 
of their corresponding stops. The second line expresses the 
ratio which the stop diameter bears to the focal length (F) of 
the camera. 

This table is based on a plate requiring' i second's exposure 
when using stop F/22 (the diameter of this stop is ^ of the focal 
length of the lens) for an open landscape. The same brand of 
plate at the same date and hour and under the same conditions 



COMPARATIVE LIGHT VALUES AND EXPOSURES. 



345 



of illumination would require i| seconds' exposure for an aver- 
age landscape when using stop jF/i6. 

Taking the length of exposure at noon, from the middle of 
April to the middle of September, as unit, the corresponding 
lengths of exposures for a plate at other hours and at different 
seasons of the year are given in Scott's table: 



Hours 

A.M. 


January 
i-iS iS-3t 


February 
i-iS is-28 


March 
I-IS 15-31 


April 
I-IS 15-30 


May 
1-15 1S-31 


June 
I-IS 15-30 


Hours 

P.M. 


4 

S 
6 

7 
8 

9 

10 

II 
Noon 


3° IS 
lo 6 

S 4 
4 3-5 
3S 3 


3° IS 
lo 6 

4 4 
3 1.8 
2.S 1-8 
2.S i-8 


— 3° 

12 7 

4 3 

2.1 1.8 

1.8 1.6 

1.7 -S 
1.6 1.4 


IS 12 
6 4 
2.5 2 
1.7 1.6 
I-S 1-4 

1.3 1.2 
1.2 I.I 


3° IS 
8 6 
3 ■ 2.5 
1.8 1.7 
I-S 1-4 
1.3 1-2 
i.i I.I 
I I 


— 3° 
14 10 

S 4 
2.3 2 
I.S 1.6 

1-3 1-3 
1.1 I.I 
I I 
I I 


8 

7 
6 

S 
4 
3 
2 
I 
Noon 




December 


November 


IS-3I I-IS 
October 


15-30 I-15 
September 


iS-30 I-IS 
August 


''T^^r' 





We see from this table that the brightest hours of the day 
are between 11 a.m. and 2 p.m., the light during this time interval 
having greater actinic power than at any other hour of the day. 
Views taken between May and August at 5 a.m. or at 7 p.m., 
for instance, should be given exposures from 10 to 30 times longer 
than the same subjects would require between the hours 10.30 
A.M. and 1.30 P.M. for the same months. 

The diagram shown on Plate XCVI represents the com- 
parative lengths of exposure at different hours of the day for 
the entire year. The abscissae represent the days of the year 
from January ist to December 31st, while the ordinates give 
the comparative lengths of exposure. This diagram has been 
constructed for so-called daylight, sunlight, and skylight com- 
bined, for a station elevation of 500 feet above sea-level and at a 
northern latitude of 40°. The lower line, marked " noon curve," 
gives the comparative lengths of exposure (as ordinates) for all 
days of the year at noon. The second curve gives similar values 
for the hours 11 a.m. or i p.m., one hour from noon, etc. 



346 



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 



Plate XCVII shows similar curves of comparative exposures 
at a northern latitude of 50°. The. full-line curves correspond to 
an elevation of 5000 feet above sea-level and the dotted curves 
correspond for the same latitude, but at sea-level. 

Experience teaches that the actinic power of light-rays ema- 
nating from objects in the shade on a bright day with a deep-blue 
sky is about ten times as great as at the same hour on the same 
date, but in dark and threatening weather. A smoky atmosphere 
reduces the actinic power of light still more; it may then be from 
twenty to thirty times less than it would be on a bright clear 
day at the corresponding hour and date. On a bright day with 
thin fleecy white clouds in the sky it is even more intense, from 
two to three times greater, than on a cloudless day at the same 
hour and date. 

The following table, by Bunsen and Roscoe, gives the gen- 
eral change in the light intensity as it increases with the altitude 
of the sun ^ove the horizon : 



Altitude of the Sun (above 
the Horizon in Degrees). 


Actinic Power of the 

Light -rays of Sun and 

Atmosphere (Combined). 


Actinic Power of the 

Light-rays, Diffused by 

the Atmosphere (without 

Sunlight). 


P 


31 


31 


JO 


17. 1 


15-1 


20 


52.6 


24.7 


3° 


91.9 


31-7 


40 


122.8 


36.1 


50 


145-5 


38.1 


60 


160.7 


39 I 


70 


170.8 


39-6 


80 


176.4 


39-7 


90 


178. 1 


39-7 



Test Exposures and Trial Plates. 

Having selected an orthochromatic plate suitable for the 
work in view, the observer should make some test exposures 
to ascertain the speed of the plate combined with the color- 
screen under known conditions. The exposed test-plates should 



COMPARATIVE LIGHT VALUES AND EXPOSURES. 347 

be developed with the developer that is to be used for all sub- 
sequent exposures. A very satisfactory way to expose these 
plates is to give one plate several exposures, say four, by with- 
drawing the slide from the plate-holder a quarter of its length 
for each successive exposure. By allowing a quarter of a second 
for each exposure, the four zones, exposed on the plate will have 
received exposures from one to one quarter of a second. A 
second plate may be similarly exposed, only increasing the final 
exposure, say to one second, when the four strips will have received 
the following exposures: ij, ij, ij, and i second respectively. 
All exposures should be made with the same lens stop. 

After development of the two trial-plates, it may be found 
that the second zone of the second test-plate, the one having 
received i J seconds' exposure, may have been the correctly timed 
strip. Having noted the conditions of light and atmosphere, 
the hour, date, stop, subject, and whether the time of i^ seconds 
was given with or without the color-screen, we can, with refer- 
ence to comparative exposure and comparative light-value tables 
or idagrams, ascertain the time for correctly exposing a similar 
plate under other conditions of illumination and atmosphere 
at other hours and dates, and, if need be, using different stops. 

It may have been found by experiment that a certain cor- 
rectly timed plate strip required i| seconds' exposure with stop 
F./ii for a dark landscape at 2.30 p.m. on June 8th, and we want 
to ascertain the correct exposure time for the same plate brand> 
but using stop i^/32 and photographing an open landscape at 
3 P.M. on August 20th. 

By inspection we find under stop F/u (page 344), in the 
table of comparative exposures, for the average or dark landscape 
the exposure value f , and the corresponding exposure for stop i''/32 
under open landscape, i. Now, as our plate required i| seconds* 
exposure for stop F/i 1 and dark landscape it will require 2 
seconds for stop i^/32 and open landscape. From the diagram, 
Plate XCVII, showing the comparative exposures at different 
hours and dates, we find that if on June 8th a.t 1.30 p.m. 



348 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS 

the illumination required i second's exposure, the same illumina- 
tion on August 2oth 'at 3 p.m. would require an exposure of ij 
seconds, hence the time required for our plate and subject, on 
August 20th at 3 p.m., would be 2X1^ = 3 seconds. 

For phototopographic purposes a rather slow, double-coated 
orthochromatic plate is preferable, as it gives a wide range for 
correct exposure, is less subject to halation, and the negative will 
have the strength requisite for making good prints. 

IVf Development of Orthochromatic Dry-plates. 

All photographic plates should be carefully dusted with a 
soft camel's-hair brush, both after insertion in the plate-holders 
and again just before immersion in the developing-bath, to remove 
all dust and foreign matter from the film surface, thus preventing 
the formation of transparent irregularly shaped spots on the 
negative. Immediately after immersion in the developer all 
air-bells or bubbles should be removed from the film surface 
by gently swabbing the submerged plate with a small tuft of 
cotton. 

The plate is placed, film side up, in the developing-tray and 
the developer is at once poured over the plate from a gradu- 
ate or pouring-vessel, with a sweeping imotion to cover the entire 
plate surface simultaneously, and the tray is kept gently rock- 
ing to flow the liquid back and forth in a wave-like motion. 

When orthochromatic, double-coated non-halation plates are 
used, development should be prolonged to allow the developer 
to penetrate the several layers of the film. The high lights in 
negatives of the ordinary single-coated orthochromatic plates 
should appear after an immersion of about 30 seconds; for double- 
and triple-coated plates the developing process should be con- 
tinued up to 6 and 10 minutes. 

If the image flashes up too quickly, the plate no doubt has 
been overexposed, and it should at once be removed from the 
developer and rinsed in clear water; development may now be 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 349 

continued in a diluted developer to which a few drops of a lo per 
cent solution of bromide of potassium have been added as a 
restraining agent. An old developer may be used to advantage 
for developing overexposed plates. 

When the image is rather slow in making its appearance the 
exposure probably was undertimed, in which case the nega- 
tive will develop up too strong with clear shadows and no details. 
The latter may be brought out, in a measure, by a lengthy immer- 
sion of the plate in a diluted or old developer, the bath being 
kept at a temperature of not over 6o°- Experience and obser- 
vation will best teach when the proper stage in the development 
may have been reached and when the plate should be removed 
from the bath. The old thumb-rule, to continue development 
until the image may be dimly outlined on the back of the plate, 
would lead to overdevelopment if observed for heavily coated 
plates. The operator may be better guided by stopping develop- 
ment as soon as the " white portions " (the shadows of the orig- 
inal) of the negative begin to change and darken. Weak nega- 
tives with clear shadows are generally due to underdevelopment. 
Too much density often results from development in a developing 
solution too concentrated or too warm. 

Since ortho and isochromatic dry-plates are extremely sen- 
sitive to yellow, orange, and, to an appreciable extent, to red 
light, it becomes necessary to exercise great care in using the 
ordinary dark-room light in their manipulation. These plates 
should be exposed only to dark (" Venetian ") " ruby light," 
covered with one or two layers of " non-actinic paper " (Deni- 
son's Orange Tissue or Gold Bank Envelope Paper). 
Even under the exercise of these precautions, the plate in the 
developing-tray should be kept covered with a cardboard or 
slab of hard rubber, except when necessary to examine the progress 
of development. Only enough illumination should be permitted 
to enter the dark-room or dark-tent as is absolutely necessary 
to conduct the operations incident to change of plates and their 
development. 



350 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Nearly all manufacturers of photographic dry-plates and 
printing-papers put up special developers, recommended for use 
with their films, in the form of powders, tabloids and concentrated 
stock solutions, with full directions for their application. 
Amateurs preferring to make up the developing solutions directly 
from the chemicals had best prepare the same in the form of 
saturated stock solutions, to be mixed and diluted just before 
use as needed. In this connection it is well to bear in mind 
that dried granulated chemicals are far more active than equal 
weights of the same in crystallized form, the weight of the latter 
being partly made up of " crystallization water." For instance, 
dried granular sulphite of sodium has double the strength of the 
same chemical in crystals, and five parts of carbonate of sodium 
in the dried state have the same chemical strength as twelve 
parts carbonate of sodium crystals. 

If photochemical solutions are prepared by weights and 
measures, careful attention should be given to the relative strength 
of their component parts. Trouble in this respect, however, 
may be avoided by using a " hydrometer " (the single-degree 
hydrometer generally used for testing silver solutions is best) 
for testing the concentration of the solutions, as dried or crys- 
tallized chemicals may then be used indiscriminately. 

Stock solutions are best put up in limited quantities, as most 
photo-chemical solutions deteriorate with age. It may be noted 
here that chemical action may be increased considerably by con- 
ducting development under relatively high temperatures, and 
decreased when working under a temperature lower than 70°, 
the latter being generally recommended for the best results. 

Nearly all developing compounds are composed of two parts, 
the developing agent proper and the alkaline solution. Among 
the developing agents more generally in use we have: Ferrous 
oxalate, pyrogallic acid, hydrochinon, eikonogen, metol, rodinal, 
their various combinations, and many others, increasing in num- 
ber from year to year. They difl'er in speed, action, keeping 
qualities, density imparted to the negative, latitude permissible 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 35 1 

in the exposure of the plate, etc. After having become famiUar 
with the action of any one developer it is recommended to adhere 
to its use to insure uniformity in the resulting negatives. 

If, in a given developer, the developing agent be used in 
excess of the correct proportion, too great a contrast between 
the lights and shadows will result; the development will not be 
under control, it will progress too fast. An insufficient amount 
of the developing agent will produce a negative deficient in strength 
and lacking the qualities essential for good printing. The gen- 
eral development will be slow with the concomitant danger of 
" frilling," which is the separation of the film from the edges 
of the glass plate. 

The alkaline ingredients of the developing-solution, prin- 
cipally carbonates of sodium (" sal soda ") and potassium, bicar- 
bonate of soda, and sulphite of soda, serve to open the pores of 
the gelatino-bromide of silver emulsion, permitting a free entrance 
of the developing agent into the upper layers of the softened 
plate film, thus producing a more prompt and effective action 
on the embedded particles of the silver haloids. An excess of 
the alkaline solution tends to produce a dense -negative, imparts 
a tendency to fog the plate, and often converts the film of the 
latter into a granular condition. 

The progress of development will be materially retarded if 
an insufficient quantity of the alkaline solution be used. Bicar- 
bonate of soda, being less active than carbonate of soda and, 
carbonate of potassium, is often used, in combination with sul- 
phite of sodium, for developing thinly coated plates to reduce 
their tendency toward fog formation and to prevent injury to 
the film. The alkaline solutions are preferably kept in hermet- 
ically closed bottles to prevent decomposition, which would soon 
take place on exposure to the air. Old alkaline solutions, or such 
containing impure sulphite of sodium, are apt to produce yellow 
stains on the negative and feshly prepared solutions of pure 
sulphite of sodium should be used whenever possible. 

All chemicals for photographic use should be pure, and it is 



352 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

recommended to purchase those especially manufactured for 
photochemical purposes. Some manufacturers of compressed 
pharmaceutical preparations have extended the tabloid system 
to photographic preparations. The advantages of the tabloid 
form to the traveller, explorer, and to the novice in the practice 
of photography are apparent. Tabloids when prepared by dry 
compression do not readily decompose; they retain in their full- 
est energy the qualities of the various ingredients of which they 
are compounded, and they have reliability, uniformity, and 
portability in their favor, 

A. Wafer and Water Tests. 

The water used in photographic operations should be dis- 
tilled or pure and free from foreign matter. By reason of its 
great dissolving power, ordinary water, in the absence of the 
distilled article, should be boiled for some time and then allowed 
to cool before decanting it for making photochemical solu- 
tions. The following simple tests may be applied to discover 
the presence of iron, magnesia, lime, etc., in ordinary water: 
For Iron: The addition of an infusion of nutgalls to water 
will show the presence of iron by imparting a grayish 
color to the mixture. If the liquid turns blue, after the 
addition of a pinch of prussiate of potash, the presence 
of iron is unmistakable. 
For Magnesia: Reduce a certain amount of water by boiling 
to V2oth of its original weight, then dissolve a few 
grains of neutral carbonate of ammonia in this liquid. 
If a whitish precipitate is formed, after the addition of a 
few drops of phosphate of soda, magnesia will be present. 
For Lime: If two drops of a concentrated solution of oxalic 
acid be added to a glass of water, the latter will contain lime 
if a milky appearance be thus produced. 
For Alkalies: Water that will change red litmus paper, on 
immersion in the same, to blue may be considered alkaline. 



' DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 353 

For Organic Water: Water becoming turbid, after the addi- 
tion of one tablespoonful of tannin solution (i part 
tannin, 4 parts water, and i part alcohol) to a tumblerful, 
' will contain organic matter. Such water is unfit for drink- 
ing purposes, particularly if the impurities are of animal 
origin. 

For Hardness: If no change be noted after adding a few 
drops of a solution of good soap in alcohol the water is 
soft; if it becomes milky in appearance it may be con- 
sidered hard. 

For Carbonic Acid: To half a tumblerful of water add an 
equal amount of lime-water; if carbonic acid be present 
. a precipitate will be formed and the addition of muriatic 
acid will cause effervescence. 

B. Developers. 

There is a distinct relation between the intensity of the light 
that has acted upon the sensitized emulsion coating of a plate 
and the actual amount of silver that is deposited upon the plate 
under the action of the developer and which defines the density 
of the negative. The laws which control the combined effects 
of light and developer upon a photographic plate have been 
studied by many photochemists. The results of the researches 
made by F. Hurter and V. C. Driffield in England are generally 
accepted as representing the best discussion on the subject. 
Capt. E. Deville, in his work on " Photographic Surveying ", 
gives an abstract of the principal papers published by Messrs. 
Hurter and Driffield, with which every photographer should 
familiarize himself if he is desirous to obtain a knowledge of 
the laws of correct exposure and development. 

I. Developing with Ferrous Oxalate. 

Ferrous oxalate is the best developing agent for phototopo- 
graphic purposes if the exposed plates are packed away in the 
field to be developed at some later period. For the develop- 



354 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

ment of test-plates in the field where the means of transpor- 
tation are limited, and where, owing to the numerous other duties 
to be performed, dark-room operations must be reduced to a mini- 
mum, developers in the form of dry powders or tabloids will 
generally be preferred. 

Good results are obtainable with the iron developer, even 
after the exposed plate has been stored away for a long time, 
before the actual development of the same is undertaken. Fer- 
rous oxalate may furthermore be recommended, because it affects 
only those particles of the silver haloids that had previously 
been acted upon by solar light, and because the final metallic 
silver deposit on the plate shows great uniformity in color. For 
these reasons negatives "developed with the iron developer are 
particularly well suited for making enlargements by " optical 
projection." 

The products resulting from the oxidation of ferrous oxalate, 
after an exposed plate has been in the developer a short while, 
exercise a restraining influence over the progress of development, 
without, however, stopping it altogether. The plate continues 
to gain density under the prolonged action of the developer, 
but the energy of the latter is held under control and the progress 
of development becomes more and more retarded under the 
gradual advance of the process of oxidation. The details of 
the image slowly become visible on the plate under the restrained 
action of the developer, gradually gaining strength and density, 
and at full development, when the plate is removed from this 
bath, the plate coating will have undergone a permanent change, 
inasmuch as particles of the silver haloid that have not been 
acted upon by the solar rays practically will have remained 
unchanged, while those that had been acted upon will become 
reduced to free silver. The final image on the negative is formed 
by a more or less gradated series of tones, conditioned by the 
various thicknesses of metallic black silver deposits that have been 
formed on the different parts of the plate. 

For the development of the plates obtained in connection 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 355 

with the Canadian surveys, Captain E. Deville uses freshly pre- 
pared ferrous oxalate compounded in two stock solutions after 
the following formulas: 

A. 
Metric Weight. Apothecaries* Weight. 

30 grammes. . . Oxalate of potash i oz. 

90 c.c Distilled water (hot) 3 oz. 

I gramme. . . . Bromide of potassium 15 grains 

§ c.c Acetic acid 10 minims 

B. 

30 grammes. . . Sulphate of iron i oz. 

60 c.c Distilled water (hot) 2 oz. 

J c.c Acetic acid 2 minims 

These stock solutions, A and B, keep well if bottled sepa- 
rately; they should be mixed for immediate use only. For the 
normal developer take to each ounce of solution A two drachms 
of solution B. The plates are developed, a dozen at a time, in 
grooved hard-rubber boxes, in which they are placed in an upright 
position. 

The formulae for Dr. Eder's ferrous oxalate developing-bath 
are as follows: 

A. 

Metric Weight. Apothecaries' Weight. 

200 grammes . . Neutral oxalate of potassium 6| oz. 

800 c.c Distilled water (hot) 26| oz. 

This stock solution should be acidulated with oxalic acid, 
adding one gramme for every 30 c.c. of the solution. 

B. 

Metric Weight. Apothecaries' Weight. 

100 grammes. . Protosulphate of iron (crystals) 3J oz. 

300 c.c Distilled water (hot) 10 oz. 

J c.c Sulphuric acid 5 minims 

Mix in the order given, adding the acid last. These solutions 
are good keepers when bottled separately, and they should be 
mixed (cold) for immediate use only. For the normal de- 



356 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

veloper and for correctly timed exposures take volumes of 
A and i volume of B and mix in a graduate. 

(a) Restraining the Ferrous Oxalate Development. 

C. 

Metric Weight. Apothecaries' Weight. 

10 grammes. . . Bromide of potassium zj drachms 

100 c.c Distilled water 3i oz. 

By adding a few drops of this solution C, termed a re- 
strainer, to the normal iron developer, given above, the 
development of the latent image will be kept under control. A 
moderate overexposure of a plate may thus be neutralized by 
adding from five to ten drops of solution C to the ferrous 
oxalate developer, which will check the general progress of develop- 
ment sufficiently to impart density to the plate and to allow the 
details to appear in the high lights before the- shadows become 
overdeveloped. 

The simultaneous appearance of lights and shadows on the 
plate (" the flashing-up " of the image) immersed in the develop- 
ing-bath would be indicative of overexposure, and the plate 
should be immediately removed from the developing-tray, be 
well rinsed in soft running water and be subjected to one of the 
following means of " retarding " development: 

1. Reducing the sulphate of iron solution (against the " nor- 

mal " amount of oxalate of potash solution) ; 

2. Reducing the temperature of the "normal developer"; 

3. Increasing the bromide of potassium (or other bromide 

salt used) ; 
For the development of overexposed plates it is advisable 
to withhold about 30 c.c. (i oz.) of solution B in a separate 
graduate and add from 2 to 4 c.c. (30 to 60 minims) of solu- 
tion C gradually pouring enough of this mixture to the de- 
veloper in the tray to produce the desired density in the plate. 
Additions to the developer are preferably made after the latter 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 357 

has been poured off the plate into a pouring vessel, flooding the 
plate with the modified developer homogeneously mixed. 

4. Diluting the " normal developer " with water; 

5. Using an old (already used) developer. 

(b) Accelerating the Ferrous Oxalate Development. 

The appUcation of so-called " accelerators " overcomes, in 
a measure, the effects of underexposure. They may also hz of 
value when developing plates representing subjects of great 
contrasts, or in all cases where the normal developer would 
produce too harsh and too dense a negative. The following 
means for accelerating development may be employed. 

1. Increasing the sulphate of iron solution (against the 

"normal" amount of oxalate of potash solution); 

2. Increasing the temperature of the "normal developer"; 

3. Using a freshly prepared and slightly concentrated solution 

of the " normal developer "; 

4. Adding a very little hyposulphite of sodium to the " normal 

developer." 

About three ounces of a developer, when mixed ready for 
use, will suffice for the development of a 4X5, and four ounces 
will be required for a 5X8 in. plate. When several plates are 
to be developed it is best to prepare a larger quantity of the nor- 
mal developing mixture at one time and develop a dozen plates 
at once. 

After the proper stage in the developrgient has been reached, 
the plate should be well rinsed in clear running water, then to 
be placed in the following so-called " clearing solution," which 
serves to prevent the precipitation of iron from the developer 
into the upper layer of the plate film. 

D. 

Cleaeing Solution. 
Metric Weight. AiMthecaries' Weight. 

150 c.c Saturated solution of alum 5 oz. 

4 c.c Citric acid i drachm 

150 c.c Distilled water , S °^- 



358 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The negative is submerged in this bath from three to five 
minutes, after which it is again rinsed in clear running water to 
remove any deposit that may still adhere to the film surface, 
to be finally placed in the " fixing-bath." 

2. Pyro Developer. 
A good " pyro " developer may be made in two solutions: 

A. 
Alkaline Solution. 

Metric Weight. Apothecaries' Weight. 

360 c.c Distilled water 60 02. 

30 c.c Carbonate of sodium (crystals) 5 oz. 

60 c.c. Sulphite of sodium (crystals) 10 oz. 

To prepare this solution with the "hydrometer," mix equal 
parts of: 

Carbonate of sodium solution, testing 40 degrees 

Sulphite of sodium solution, testing. ,. 80 degrees 

B. 
Pyro Solution. 

Metric Weight. Apothecaries' Weight. 

4 c.c Sulphite of sodium (crystals) i drachm 

180 c.c Distilled water 6 oz. 

After the sulphite of sodium has been dissolved in the 6 oz. 
of water, add acetic acid to this solution until the liquid turns 
blue litmus paper red, then add: 

Metric Weight. Apothecaries' Weight. 
30 c.c Pyrogallic acid i oz. 

For the " normal developer " mix: 

Metric Weight. Apothecaries' Weight. 

4 c.c of B (pyro solution) i drachm 

60 c.c. , of A (alkaline solution) 2 oz. 

For winter use, dilute this with 60 c.c. (2 oz.) of tepid dis- 
tilled water, whereas for summer use, dilute with 90 to 150 c.c. 
(3 to 5 oz.) of cold distilled water. 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 359 

Both solutions should be kept in well-stoppered bottles. If 
the negatives show yellow stain a new solution A should be made 
or a freshly prepared sulphite of sodium should be used. 

A smaller quantity of sulphite of sodium in solution A will 
produce a warmer tone, a larger quantity a grayish-blue to bluish- 
black tone. An increase of A in the normal developer mixture 
may bring out detail in an underexposed negative. If the high 
lights in the negative are flat more of the pyro solution (B) may 
be used, if they are too intense less may be used. 

If too Kttle of solution B be used the alkali will be in excess 
and a foggy negative may be the result. 

Pyrogallic acid, being a strong poison, should be carefully 
handled, clearly labeled, and securely stored. 

3. Metol Developer 

may be made in either one or two solutions, both keeping well. 
The two-solution developer, however, is preferable, as it not only 
gives a better control over the progress of development but it 
also gives the means for developing overexposed plates by 
adding a little of solution B to the " normal developer," or by 
using solution A alone (diluted) if the plate was greatly over- 
exposed. 

A. 

Metric Weight. Apothecaries' Weight. 

1000 c.c Distilled water 10 oz. 

15 grammes.. . . Metol 75 grains 

120 grammes.. . . Sulphite of sodium (crystals) ij oz. 

Dissolve the metol in water before adding the sulphite of 
sodium. 

B. 

Metric Weight. Apothecaries' Weight. 

1000 c.c Distilled water 10 oz. 

150 granunes. . Carbonate of soda (crystals) i . 75 oz. 

1.5 grammes. . Bromide of potassium 8 grains 



360 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

For the normal developer take: 

Solution A, I volume 

B, I " 
Water, i " 

Metol developer may be used repeatedly. An old developer 
is to be recommended for overexposures. If the plale shows 
a tendency to fog add from 10 to 20 drops of a 10 per cent solu- 
tion of bromide of potassium to the developer. 

For correct exposures the image should appear in detail within 
from 4 to 10 seconds and development should be complete in 
4 or 5 minutes. 

As the density of the plate is somewhat reduced in the fixing- 
bath development should be carried on a little further than one 
would otherwise do. 

For underexposed plates the normal developer should be 
diluted. 

4. Metol Bicarbonate Developer 

is to be recommended for its excellent keeping qualities and uni- 
form results. It may be used repeatedly without materially 
affecting the general progress of the development. The bicar- 
bonate of soda makes this developer very safe in action, prevent- 
ing injury to the film and fogging of the plate. 

Metric Weight. Apothecaries' Weight. 

10 grammes. . . Metol i oz. 

600 c.c Distilled water 60 oz. 

Thoroughly dissolve the metol in the water and then add: 

Metric Weight. Apothecaries' Weight. 

60 grammes. . . Sulphite of soda (crystals) 6 oz. 

30 grammes. . . Bicarbonate of soda 3 oz. 

To prepare this developer with the hydrometer, mix: 

Metric Weight. 

300 c.c (30 oz.) Sulphite of soda solution testing. ... 75 dee. 

300 c.c f 30 oz.) Bicarbonate of soda solution testing. . 50 deg. 

10 c.c ( I oz.) Metol dissolved in 120 c.c. (12 oz.).. . Water 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 361 

5. Hydrochinon Developer. 
A. 

Metric Weight. ' Apothecaries' Weight. 

600 c.c Distilled water (hot) 20 oz. 

120 grammes. . . Sulphite of soda (crystals) • 4 oz. 

4 grammes. . . Sulphuric acid i drachm 

23J grammes. . . Hydrochinon 360 grains 

2 grammes. . . Bromide of potassium 30 grains 

Diluted with enough water to make up to 

Metric Weight. Apothecaries' Weight, 
960 C.c 32 oz. 

B. 

60 grammes. . . Carbonate of potash 20 oz. 

60 grammes. . Carbonate of soda (crystals) 2 oz. 

With enough water to make up to 

Metric Weight. Apothecaries' Weight. 
960 c.c 32 oz. 

C ("Accelerator"). 

30 grammes. . . Caustic soda i oz. 

300 grammes. . . Water 10 oz. 

D ("Restrainer"). 

14 grammes. . . Bromide of potassium J oz. 

150 c.c Water S oz. 

For the normal developer take 

Metric Weight. Apothecaries' Weight. 

30 c.c Solution A i oz. 

25 c.c " B f oz. 

120 c.c Water 4 oz. 

The working temperature of this developer should not vary 
much between 65° and 75°. 

For underexposure add a few drops of solution C to the normal developer 
" overexposure " " " " " " D" " " " 



362 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

More of solution A (as given for the " normal developer ") 
will increase the density and more 'of solution B produces an 
increase in detail. 

Should the . negative, after development with hydrochinon, 
show yellow stain, it may be cleared and intensified, if need be, 
by immersion in the following bath: 

E. 

Bichromate of potassium 10 parts 

Hydrochloric acid 10 " 

Water 1000 ' ' 

The stained negative is kept in this solution until it appears 
completely bleached, when it should be well rinsed in running 
water. If the bleached negative be now developed anew no 
trace of fog will appear; redevelopment should be carried on 
until the desired strength and density may be attained. 

The following hydrochinon developer is recommended by 
L. E. Jewell for photographing clouds with a ray-filter: 

A. 

Metric Weight. Apothecaries' Weight. 

30 c.c Hydrochinon i oz. 

150 c.c Sulphite of sodium (crystals) 5 oz. 

750 c.c Distilled water (hot) 25 o^- 

7 c.c Alcohol (95%) J oz. 

After the sulphite of sodium has been dissolved in hot water 
add the hydrochinon and shake well. Filter the solution, add 
the alcohol, and again shake well. 

B. 

Metric Weight. Apothecaries' Weight. 

30 c.c Carbonate of potassium i oz. 

30 c.c Ferrocyanide of potassium 1 oz. 

360 c.c Distilled water 12 oz. 

The ferrocyanide of potassium acts as an accelerator for the 
hydrochinon. For the normal developer take 

Metric Weight. Apothecaries' Weight. 

90 c.c Solution A 3 oz. 

30 c.c " B loz. 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 363 

adding from 6 to lo drops of a lo per cent solution of bromide 
of potassium to this mixture. 

Development may be begun with the " normal developer " 
and if signs of over or underexposure are noted the developing- 
bath should be modified to the following mixtures: 

For overexposure: 3J volumes sol. A; 

I volume sol. B; 

J volume of a ten per cent bromide of potassium solution. 
For underexposure: 3 volumes sol. A; 

I volume sol. B, omitting the bromide of potassium sol. 

In changing from one of these developers to the other the 
plate had best be rinsed in clear water, although this would 
not be necessary when changing the plate from the underexpos- 
ure bath to the normal developer, nor when transferring the 
plate from the normal to the overexposure bath. 

6. Metol Hydrochinon Developer. 
For use in winter, dissolve in the order given: 

Metric Weight. Apothecaries' Weight. 

7 grms Metol J oz. 

7 grms Hydrochinon ^ oz. in 

2400 c.c Distilled water 80 oz., then add 

120 grms Sulphite of soda (crystals) 4 oz. 

75 grms. .... Carbonate of soda (crystals) 2J oz. 

To prepare this solution with the hydrometer, mix in the 
order given: 

Metric Weight. 

600 c.c (20 oz.) Sulphite of soda solution testing. . . .60 deg. 

600 c.c (20 oz.) Carbonate of soda solution testing. .30 deg. with 

7 grammes (J oz.) metol and 7 grammes (i oz). hydro- 
chinon dissolved in 
1200 c.c (40 oz.) Water. 

For summer use this normal developer should be diluted with 
an equal quantity of water to keep the development under good 
control. If the negatives show too much contrast less hydro- 
chinon and more metol may be taken. 



364 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

7. Bromo-Hydrochinon Developer. 

Bromo-Hydrochinon Developer recommended for developing 
overexposed plates and for producing density in the negative. 

A. 

Metric Weight. Apothecaries' Weight. 

750 c.c Distilled water (hot) 25 oz. 

90 grammes. . . Sulphite of soda (crystals) 3 oz. 

15 grammes. . . Hydrochinon J oz. 

7 grammes. . . Bromide of potassium i oz. 

B. 

750 c.c Distilled water 2S oz- 

180 grammes. . . Carbonate of soda (crystals) 6 oz. 

For the normal developer take equal volumes of A and B. 
If the plate shows signs of underexposure it should be immersed 
in a freshly prepared and diluted developer, and if sufficient 
detail does not appear in this bath, the plate should be removed 
to another tray containing water to which a little of the alkaline 
solution (solution B) has been added, leaving the plate in 
this bath as long as an increase in detail may be noted. If 
still weak, development may be finished in a fresh developer. 

8. EiKONOGEN Developer. 
A. 

Metric Weight. Apothecaries' Weight. 

15 grammes. Eikonogen 1 oz. 

60 grammes . Sulphite of sodium (crystals) 4 oz. 

0.3 gramme. . Bromide of potassium. . . .' 10 grains 

goo c.c Distilled water 60 oz. 

B. 

45 grammes. . . Carbonate of soda 3 oz. 

300 c.c Distilled water 20 oz. 

For the normal developer take 3 parts of solution A and i 
part of solution B, adding i drop of a 10 per cent solution of 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 365 

bromide of potassium to each 30 c.c. (each oz.) of mixed 
developer.* 

If the " acid fixing-bath " be used after development with 
eikonogen the negatives are apt to be marred by semi-transparent 
streaks, t 

9. "Eiko-Cum-Hydro" Developer. 
A. 

Metric Weight. Apothecaries' Weight. 

600 c.c Distilled water (hot) 20 oz. 

120 grammes. . . Sulphite of soda (crystals) 4 oz. 

22 grammes. . . Eikonogen 330 grains 

10.5 grammes. Hydrochinon 160 grains 

adding enough water to make up to 

Metric Weight. Apothecaries' Weight, 
960 c.c 32 oz. 

B. 

600 c.c Distilled water 20 oz. 

60 grammes. . . Carbonate of potash 2 oz. 

60 grammes. . . Carbonate of soda (crystals) 2 oz. 

adding enough water to make up to 

Metric Weight. Apothecaries' Weight, 
960 c.c 32 oz. 

* For developing bromide paper prints add 2 parts distilled water to the normal 
developer as given here. The developer should be renewed after each 4 to 6 
prints have been developed. 

t The following "fixing-solution" should be used with this developer for both 
plates and bromide prints, as it prevents all possibility of the developer staining 
the negative: 

Metric Weight. Apothecaries' Weight, 

60 grammes Hyposulphite of soda 4 oz. 

15 grammes Bisulphite of sodium i oz. 

300 c.c Distilled water 20 oz. 

This fixing-solution remains colorless after repeated use. 



366 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

For the normal developer take 

Metric Weight. Apothecaries' Weight. 

30 c.c Solution A i oz. 

15 c.c Solution B 4 drachms 

go c.c Distilled water 3 oz. 

More of solution A would increase the density of the nega- 
tive and more of solution B would tend toward an increase in 
detail. 

10. Amidol Developer. 

Metric Weight. Apothecaries' Weight. 

600 c.c Distilled water 20 oz. 

4 c.c Sulphurous acid i drachm 

15 grammes. . . Sulphite of soda, granular, dry 4 drachms 

3 grammes. . . Amidol 46 grains 

Dissolve the chemicals in the water in the order given. With 
this developer the image should appear rather quickly with full 
intensity and a wide gradation of tones. 



C. Fixing the Negative. 

After removal from the " clearing-bath," the negative is 
placed in the " fixing-bath " to preserve the developed image, 
to render the negative light-proof. 

After the proper stage in the development of the plate has 
been reached, by immersion in one of the preceding developers, 
in the dark-room, the negative should be well rinsed in clear 
running water to remove all traces of the developing and clearing 
solutions. It may next be placed in the " clearing-bath " given 
under " iron developer " (ferrous oxalate), solution D. This 
bath should not be omitted after development with ferrous 
oxalate, but it may be omitted when using most of the other 
developers. After removal from the clearing-bath the negative 
should again be rinsed in clear water before it is subjected to the 
" fixing-bath." 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 367 

Metric Weight. Apothecaries' "Weight. 

4 c.c Sulphuric acid i drachm 

480 grammes. . . Hyposulphite of soda 16 oz. 

60 grammes . . . Sulphite of sodium (crystals) 2 oz. 

30 grammes . . . Chrome alum * i oz. 

1920 c.c Distilled water (warm) 64 oz. 

This acid fixing-bath should be mixed in the following order: 
Dissolve the hyposulphite of soda (i6 oz.) in 1440 c.c. (48 oz.) 
of warm distilled water, the sulphite of sodium crystals (2 oz.) 
in 180 c.c. (6 oz.) water. Next dilute the sulphuric acid 
(i drachm) with 60 c.c. (2 oz.) water and pour this slowly 
into the sulphite of sodium solution and add this to the hypo- 
sulphite of soda solution. Now dissolve the chrome alum (i oz. 
in summer and J oz. in winter) in 240 c.c. (8 oz.) of warm 
distilled water and add this solution to the bulk of the mixture, 
when the fixing-bath (after cooling) will be ready for use. 

This fixing-solution is a good keeper and will not discolor 
until after repeated use. It clears the shadows of the negative 
and hardens the film, thus materially preventing " frilling " 
(separation of film edge from glass surface) in the final washing. 

The negative should be kept in this bath until the last trace 
of the milky-white appearance of silver bromide, when examined 
from the back of the negative, has entirely disappeared and the 
shadows have become perfectly transparent. This should require 
an immersion of about 5 minutes. The negative has now become 
light-proof and it should be thoroughly rinsed in clear running 
water for at least one half hour, or when the water is cold, for 
fuUy one hour, to free the film from any trace of the hyposul- 
phite. Before removal from the water the film surface should 
be swabbed with a wad of cotton, again rinsed, and finally be 
placed in rack to dry 'spontaneously. If no running water is 
available the washing may be done in ten to fifteen changes 

* The given amount is for use in summer. In winter only 15 grammes 
(4 drachms) chrome alum should be taken. An excess of alum may cause a 
precipitate to form on the negative, imparting a mottled appearance to the latter. 



368 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

of water, transferring the negative from one tray to the other 
and refilling each tray with fresh water after passing the nega- 
tive from tray to tray at intervals of five to ten minutes. 

It is most important that every trace of hyposulphite of soda 
be eliminated in the final washing to impart keeping qualities 
to the negative. Any subsequent formation of a crystallized 
coating of the film, followed by a gradual fading of the image, 
may generally be regarded as the direct result of an imperfect 
removal of the hyposulphite of soda in the final washing. 

I. Tests for Presence of HYPOsuLpmTE of Soda. 

To ascertain whether the hyposulphite of soda may have been 
thoroughly removed from the film of the negative the following 
tests may be applied. 

The simplest test for presence of " hypo " in the last washing 
is made by adding a few drops of the following solution (Prof. 
Boettcher's test) to a little of the water drained from the negative : 

Metric Weight. Apothecaries' Weight. 

0.2 gramme .... Permanganate of potash 3 grains 

I gramme Caustic soda 15 grains 

460 c.c Water 16 oz. 

If " hypo " be still present the red color of the mixture will 
be changed to green. 

This solution should be kept in a well-stoppered bottle incased 
in a light-proof wrapper or box. This test is generally con- 
sidered very satisfactory, as the quantity of " hypo " left" in the 
pores of the film must be very small indeed if the pink or red 
colored solution does not change color within a few minutes after 
mixing the drainings from the plate with the permanganate solu- 
tion. 

Another test is as follows: 

Beat up a piece of starch, about the size of a pea, with } oz. 
of water and boil in a test-tube to a clear jelly. To this add 



DEVELOPMENT OT ORTHOCHROMATIC DRY- PLATES. 369 

one drop of tincture of iodine, which will produce a dark-blue 
color. Now fill another test-tube with the drainings from the 
negative and add one drop of this blue solution, stirring the 
mixture well. If " hypo " be present the blue color will be dis- 
charged. 

Should the final washing of the negative have to be cut short, 
on account of " frilling of the film," during warm weather and 
in the absence of ice, or should it have to be interrupted for any 
other reason, it is recommended to place the negative in a freshly 
prepared " clearing-bath " (solution D, given under ferrous oxa- 
late developer), or in a i : 30 solution of bromine in water, to 
neutralize and destroy any " hypo " that may still be retained 
in the surface layer of the film. 



2. Drying the Fdhshed Negative. 

Negatives are best dried in a cool, dry atmosphere, prefer- 
ably under a mild draft. In a warm, damp, or wet climate the 
drying of the finished negative would proceed too slowly, greatly 
increasing the density of the film, particularly toward the center 
of the plate, which generally dries last. Under such adverse 
conditions the drying of the films may be accelerated by flowing 
proof alcohol over the film a few times before placing the nega- 
tive in the rack to dry. Heat, however, should never be applied 
to the negatives for purposes of drying, as it liquidizes the soft 
gelatine coating. Great care should be exercised not to inter- 
change the trays and vessels used for the various photochemical 
solutions. 

3. Intensification op a Negative with the Aid of Metallic Salts. 

With correct exposure and development intensification need 
not be resorted to. Light somewhat retards the process of inten- 
sifying, and it is advisable to conduct the operation in the dark- 
room, or at least in subdued light. A negative that had been 



370 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

dried after fixing is more readily acted upon by the intensifier 
than one that has just been removed from the washing-tank. 

Should the final negative be too thin for good printing it 
may be intensified, after thoroughly washing to eliminate all 
traces of " hypo," as previously noted, by subjecting it to the 
following treatment: 

Pour a sufficient quantity of a saturated solution of bichloride 
of mercury in water into a solution of 37 grammes (ij oz.) iodide 
of potassium in 180 c.c. (6 oz.) water until the point be reached 
when the forming red precipitate can no longer be dissolved by 
shaking. No more bichloride of mercury solution should be 
added than just enough to make the solution a slight shade turbid. 

Now add 28 c.c. (i oz.) hyposulphite of soda, dissolve, and 
add enough water to make up 600 c.c. (20 oz.) of this inten- 
sifier. Just before use dilute one part of this intensifier with 
three parts of water. The weak negative is submerged in this 
diluted solution, exercising care not to carry the intensification 
too far, although the negative may be reduced in a measure 
by again placing it in the fixing-bath for a short while. If the 
negative was not perfectly " fixed " before subjecting it to the 
action of the intensifier it will be marred by a yellow stain. 

4. Intensipication with Silver Cyanide. 

After having washed the weak negative thoroughly for half 
an hour in clear running water it should be immersed for ten 
minutes in a five per cent solution of alum and again thoroughly 
washed before applying this intensifier. 

A. 

Metric Weight. Apothecaries' Weight. 

16 grammes. . . . Bichloride of mercury 240 grains 

16 grammes. . . . Chloride of ammonia 240 grains 

600 c.c Distilled water 20 oz. 

B. 

16 grammes. . . . Chloride of ammonia 240 grains 

600 c.c Distilled water 20 oz. 



DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 37 1 

C. 

Metric Weight. Apothecaries' Weight. 

4 grammes .... Nitrate of. silver 60 grains 

60 c.c Distilled water 2 oz. 

This nitrate of silver solution is poured, while stirring, into 
the following solution: 

Metric Weight. > Apothecaries' Weight. 

4 grammes .... Cyanide of potassium, C.P 60 grains 

180 c.c Distilled water 6 oz. 

The cyanide of potassium serves as a fixing-agent; it is a 
very strong poison and the solution C should be labelled " poison " 
and carefully stored. 

To intensify the negative enough of solution A is flowed 
over it to completely submerge the film. It is subjected to this 
bath sufiiciently long to either partially or completely whiten 
the film, according to the degree of density desired. After 
removal from this bath the plate should be carefully rinsed and 
immersed in solution B for one minute,, again be rinsed and 
then be placed in the cyanide solution (C), where it is kept until 
the color of the film is changed to a dark brown or black, when 
it should be removed, thoroughly washed in running water, and 
placed in rack to dry. The solutions A and B should be thrown 
away when once used, while the cyanide solution (C) may be 
returned to its bottle to be used again. 

5. Prof. R. E. Liesegang's Intensifer. 

Prof. Liesegang recommends the following solution for the 
intensification of underdeveloped negatives: 

Apothecaries' Weight. 

r Sulphate of copper 75 grains 

Sol. I < Bromide of potassium 75 " 

I Water 6i oz. 



Sol. II 



J Nitrate of silver 91 grains 

I Water 4 oz. 



372 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

After the negati\'c, which may be found too thin to print 
well, has been washed from three to five minutes in running 
water to remove every trace of hyposulphite of soda (see Prof. 
Boettcher's " hypo " test) it is placed for ten minutes in solution 
I until thoroughly bleached. The longer the negative remains in 
this bath the greater the final density will be; therefore this pari 
of the process should be carefully watched. Still, should the 
intensification be too marked, the density may again be reduced 
with the usual fixing-bath of " hypo." 

After removal from solution I the negative is again rinsed in 
running water, or washed in five changes of water for ten minutes, 
and it is then placed in the nitrate of silver solution until it becomes 
thoroughly blackened (no white spots should be apparent when 
viewed from the back). The intensified negative is now washed 
for at least one hour in frequent changes of water; in running 
water half an hour will probably suffice. 

A negative that had already been dried after fixing is more 
];eadily acted upon than one that has just left the washing-tank. 
Light retards the process materially and intensification is best 
conducted in the dark-room, or at least in subdued light. 

6. Intensifying Negatives without the Use of Metallic Salts. 

After a thorough washing in running water the negative is 
immersed in the following solution: 

Metric Weight. Apothecaries' Weight. 

0.3 gramme Potassium bichromate 5 grains 

0.6 gramme Potassium chloride 10 grains 

0.25 c.c Hydrochloric acid 4 minims 

30 c.c Distilled water. .' i oz. 

Under the action of this solution the silver deposit on the 
negative is converted into chloride of silver. The plate is retained 
in this bath until the image appears well bleached, when it is 
removed and thoroughly washed in clear running water to elimi- 
nate the chromium salts from the film. The negative may 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 373 

now be redeveloped in any developer. Some operators recom- 
mend soaking the plate in a dilute solution of sulphurous acid 
or acid metobisulphite to facilitate the elimination of the chro- 
mium salts in the washing between the bleaching and redeveloping. 
Pyro-soda, pyro-ammonia, metol, and pyro-metol developers 
give a considerable increase in density when used for redevelop- 
ment vnth this method of bleaching the image of a weak negative 

7. REDtJCING THE DENSITY OF A NeGATTVE. 

BrowTi and yellowish stains and also a certain iridescence 
of the film surface may all be caused by having the developing, 
bath too warm, too strong in alkali for the plate, or by having 
used the plain " hypo " solution in fixing the negative. The 
same defects may also be caused by using too old a solution in 
the fixing-bath, or when the latter has been used too often, and 
sometimes, too, by an insufficient fixing of the negative. A weak 
solution of perchloride of iron vsdU remove the yellow stains 
when this bath is followed by an immersion in the acid fixing- 
bath. 

Density in a negative, brown stains, and the metallic irides- 
cence may aU be removed by applying the following " reducing 
solution." Dissolve 

I part of red prussiate of potash in 
15 parts of water. 

Wrap the bottle containing this solution in yellow paper, to 
delay decomposition of the solution by the effects of the light, 
then dissolve 

I oz. hyposulphite of soda in 
15 oz. water. 

Add from one half to one ounce of the red prussiate solution 
to the above hyposulphite of soda solution immediately before 
use and place the negative in this bath directly after fixing. 



374 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

A dry negative should first be soaked in water for a few minutes. 
Watch the negative carefully while it is in this reducing solution, 
rocking the tray and avoiding strong light during the immersion, 
and remove it to running water at once when it has been suffi- 
ciently reduced or cleared. 

Great care should be exercised to keep the trays and the 
vessels as clean as possible and never to interchange them when 
developing and fixing plates. All vessels used for developing 
should never be used for any other purpose and the fixing-tray 
should never be used for developing. 

8. Cooling-solutions. 

To accelerate the drying of the films in a damp or wet 
climate the plates may be immersed, just after the final wash- 
ing, in a bath composed of equal parts of water and alcohol 
before they are placed in the rack to dry. All negatives, when 
dried slowly in a damp and warm atmosphere, will become more 
intense than when dried spontaneously in a draught or in a cool 
current of air. 

Should the conditions be unfavorable enough to require 
artificial heat for drying the films, it is recommended to immerse 
the negatives from six to ten minutes in a solution of one part 
of Woodbury Antipyr in ten parts of water, after which they 
are rinsed and placed in the rack to dry. This solution may 
be used repeatedly. 

When the development of plates has to be carried on during 
very warm weather in localities where neither ice nor cold water 
is obtainable, the several baths may be kept cooled by setting 
their trays in shallow vessels that are filled with some .cooling- 
solution, of which we may enumerate the following: 

1. One part of nitrate of sodium and four parts of water. 

2. One part nitrate of ammonia and one part water. 

3. One part sulphocyanate of potassium and one part water. 

4. Three parts nitrate of sodium and four parts water. 



DEVELOPMENT OF ORTHOCHROMATIC DRY-PLATES. 375 

5. One part chloride of potassium and four parts water. 

6. Three parts sulphate of sodium and two parts diluted 

nitric acid. 

7. Nine parts phosphate of sodiimi and four parts dilute 

nitric acid. 

8. One part sal ammoniac, one part saltpeter, and one part 

water. 

9. Five parts sal ammoniac, five parts saltpeter, and sixteen 

parts water. 

10. Eight parts sulphate of sodium and five parts concen- 

trated sulphuric acid. 



D. Negative Varnish. 

For a better preservation of the negative, its film may be pro- 
tected by a thin and uniform coatiug of varnish, to be applied 
after the film has become thoroughly dried and hardened. The 
dr}' negative should be carefuUy dusted and held near a fire until 
it is uniformly warm. It is now balanced on the finger-tips of 
the left hand, film side up, and a small portion of the varnish 
is poured on the film surface, gradually turning and tipping the 
plate to cause the varnish to flow to each corner, covering the 
entire plate but not going over the same place twice. The 
plate is now kept warm, still holding it in a horizontal position, 
under a gently rocking motion, until the varnish has dried with- 
out leaving fines or ridges. 

A colorless and transparent varnish may be made by dis- 
solving one ounce soft copal in ten ounces benzine. 

A good negative varnish that will permit of retouching the 
negative may be prepared after the following formula; 

Metric Weight. Apothecaries' Weight. 

10 graimnes Amber powder (melted) 150 grains 

6 grammes Unvulcanized rubber 90 grains 

1750 c.c Chloroform 50 drachms 

1750 c.c Benzole 50 drachms 



376 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

The proportion of benzol added determines the degree of 
"' mat " that may be imparted to the dry coating of this varnish. 



V. Photographic Printing. 

Photographic printing is the process of obtaining positive 
copies from a negative, on specially prepared paper (or other 
sensitized material), by means of light transmitted through the 
negative. 

The printing process is as follows: 

Place the printing-frame, face downward, on a table and 
remove the backboard. Lay the negative, which should be 
perfectly dry, film side up, in the frame and on this place a 
sheet of sensitized paper, face down, in contact with the film. 
This paper may be covered with a blotting-paper or piece of 
thin felt, after which the back of the frame is replaced and clamped 
in position. The frame should how be carefully turned over, 
and any spots that may be on the glass surface should be removed 
before exposing the negative to the light. These operations may 
be conducted in the subdued light of a room when using so- 
called " printing-out " papers, but even this paper should not 
be exposed too long to such light and not at all to the direct rays 
of any light. 

The progress of the printing should be examined occasion- 
ally, in subdued light, by opening one side of the backboard 
of the frame and raising one end of the paper. The printing 
should be a trifle darker than is desired for the final picture, 
and darker when toning for blue-black tones than when toning 
for warm-brown tones. If not sufficiently printed the back- 
board is again closed and carefully latched. Only one side of 
the back of the frame should be loosened for examination of 
the print to maintain perfect registry between the paper and 
the negative. 

Weak negatives should be printed in diffused light by covering 
the printing-frame with a sheet of tissue-paper (or with several 



PHOTOGRAPHIC PRINTING. 377 

thicknesses if required). Strong and dense negatives are best 
exposed to direct sunlight. It is important to print a shade 
deeper than required for the finished picture, ks the print will 
always bleach somewhat in the toning and fixing process. 

Should the whites darken before the shadows become bronzed, 
when printing in direct sunlight, it may be taken as an indication 
that the negative is too weak for printing in intense light and its 
prints should be made in the shade or in diffused light. 

Should the shadows be fully bronzed before details in the 
high lights appear, when printing in diffused light, it is a sign 
that the negative is sufficiently dense to require printing in sun- 
light. 

To reduce overprinted pictures an old fixing-solution to 
which a few drops of a saturated solution of ferrocyanide and 
ammonia have been added may be recommended. 

Prints sufficiently exposed may be collected in a light-proof 
drawer or in a dark receptacle until a number are ready for toning. 
There is a large variety of printing-papers in the market for 
obtaining positive copies, from negatives, and special directions 
for their use and manipulation accompany each brand. They 
may be divided into two groups. The first, comprising all papers 
requiring special developing to bring out the latent image, are 
known as develo ping-out papers. These undergo the same 
process as a photographic plate after exposure to bring out and 
fix the image. 

The second group comprises all so-called printing-out 
papers (mostly gelatine or collodion papers), the image becoming 
visible during the printing. 

The papers of the first group (" developing-out " papers) 
are mostly very sensitive and the actinic action of the light on 
their sensitive silver salt is so rapid that these papers should 
be manipulated in the dark-room only. The printing, too, is 
best done in the dark-room with artificial light. These papers 
have the advantage that they give ready means for obtaining 
prints at any time irrespective of the weather, and most of them 



378 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

are invaluable for enlarging purposes, enabling the operator to 
make prints of almost any size from relatively small negatives. 
The so-called bromide, platinum, and platino- bromide papers 
are well suited for making direct enlargements from photo- 
topographic negatives. Such enlargements are preferably 
used in iconometric plotting in place of the small contact 
prints. 

The papers of the second group are of special value for mak- 
ing contact prints in the field; they are less sensitive to the light 
than the " developing-out " papers and only require toning 
and fixing after removal from the printing-frame. We find a 
large variety of these papers in the market, with special direc- 
tions for their manipulation and use. 

The older and still popular " printing-out " paper known 
as " silver-printing," " albumenized " or " sensitized albumen " 
paper is not as sensitive as most of the modem " printing-out " 
papers. It is coated with albumen and sensitized with a solu- 
tion of nitrate of silver. 

Shortly before placing this paper in the printing- frame it 
should be fumigated with ammonia vapors, thus increasing the 
brilliancy of the prints and the sensitiveness of the" paper as well 
as reducing the time required for the actual printing and facili- 
tating the subsequent process of toning by reducing any ten- 
dency toward " blistering " (separation of the sensitized film 
from the paper). 

The fuming should be done only for immediate use of the 
paper, as it impairs the keeping qualities of the sensitized coat- 
ing before fixing. Fuming is best done in a wooden box about 
six inches or more deep and having a wooden grating supported 
about three inches above the bottom of the box. A saucer con- 
taining some " stronger water of ammonia " is placed on the 
bottom of the box, the grating is placed in position with the albu- 
menized paper laid flat upon it, and the box is now closed, expos- 
ing the paper for fifteen to thirty minutes to the ammonia vapor. 
This operation should be conducted under exclusion of light, and 



PHOTOGRAPHIC PRINTING. 379 

it is recommended to remove the paper from the box at least five 
minutes before it is placed in the printing-frame. 

As most prints have an unpleasant reddish tint when they 
leave the printing-frame they are generally subjected to the 
toning process, which converts the reddish tint into a warm 
sepia, a brown or a dark-purple tint, approaching a black color, 
according to the formula used for preparing the toning-bath 
and dependent on the length of time they were exposed in the 
printing-frame. 

A. Toning Photographic Prints. 

Papers requiring development of the latent image, of course 
need no special toning-bath, their pictures appearing under the 
action of the developer in soft and warm effects, either in black 
and gray or in black and brown tones. 

There are many formulas available and many preparations 
in the market, both for making separate toning-solutions and 
" combined toning-baths," which tone and fix the print at one 
immersion. Nearly every brand of printing-out paper is fur- 
nished with special directions and formulas for toning, fixing, 
and hardening. 

All toning-solutions contain besides gold (or platinum) an 
alkali (bichromate of soda, borax, carbonate of soda, etc.) to 
retard the action of the bath. The more gold the print may 
be made to take up, the more the gold deposit will partake of a 
ruby color and the more permanent becomes the picture. The 
final tone of the picture is conditioned by both the character 
and the quantity of the alkali used to neutralize the acidity of 
the gold solution, some alkalies (acetate of soda) giving a brown 
to purplish tone, while others (carbonate of soda) produce 
tenes closely approaching a soft black in the deeper shadows 
of the picture. For producing good brown to black tones the 
following plain gold "separate toning-bath" may be prepared: 

Dissolve 1.3 grammes or 20 grains chloride of gold in 570 



380 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

c.c. or 20 ounces distilled water and label the bottle " Gold 
solution." 

As this solution contains one grain of gold per ounce of liquid 
one may substitute one ounce of the solution in all formulas for 
toning-baths for every grain of gold given in the formula. 

Next dissolve one ounce each of acetate of soda and car- 
bonate of soda in twenty ounces distilled water, shake the mix- 
ture well and filter into a bottle. 

To prepare the toning-bath one ounce (30 c.c.) of the " gold 
solution " is added to forty-eight ounces (1440 c.c.) distilled 
water and this solution is neutralized by gradually adding of 
the acetate and carbonate of soda solution till litmus paper no 
longer changes color when dipped into this mixture. When 
cold tones are desired in the picture just enough of the alkaline 
solution should be added that red litmus paper turns blue when 
dipped into the liquid. An excess of alkali, however, has a ten- 
dency to make the prints appear more toned than they really 
are, and such prints undergo a decided bleaching in their sub- 
sequent immersion in the fixing-bath. 

Ten ounces of this toning mixture will suffice to tone about 
eight 5X8 prints. To tone more, either a larger quantity may 
be made up at once or more of the gold and alkali solutions 
may be added to the old bath. The latter method has the advan- 
tage that this toning solution may be used at once, whereas the 
freshly prepared bath should be made up about twenty-four 
hours before it is really wanted, the freshly prepared solution 
working less uniform than an older one. 

Should this bath tone unevenly or should the prints come 
out streaky, it is advised to make the bath slightly alkaline and 
diluted with water. The desired tone should be produced in 
six to ten minutes. 

The temperature of this bath should be kept rather low, 
not to exceed 60° F. 

The toning process proper is conducted as follows: 

After the final washing of the prints in running water suffi- 



PHOTOGRAPHIC I^RINTING. 38 1 

ciently long to remove all free silver, ,Qr^ in five or six changes 
of water, they are transferred, one at .a time and face down- 
ward, to the toning-bath. The tray meanwhile should be gently 
rocked and the prints kept in motion by transferring the lower 
ones to the top ' singly, keeping this process up to maintain a 
layer of liquid between the prints and to remove at the same 
time any air-bubbles adhering to the film surface, thus assuring 
an even toning for all. 

Prints first begin to tone on their surface, and if not toned 
sufficiently deep, they will turn 'a reddish brown later in the 
fixing-bath. If the original red color appears to have disappeared, 
on examining the print through transmitted light, toning may 
be stopped. 

Prints cannot be toned dark if ,the printing was not carried 
sufficiently far, and it should always be remembered that the 
original tone of the print will somewhat fade in the toning- and 
fixing-baths. Soon after immersion in the toning-bath, of which 
the composition has been given, the prints will change color to 
a dark bro~wn, then to purple, and finally to a soft black. As 
soon as the prints may have been toned to the desired shade 
they are to be fenioved to clear running water, where they may 
remain until enough are ready for the fixing-bath. 

If no running water be available the toned prints should 
first be placed for about one minute in a saline bath of one 
ounce of chloride of sodium (common salt) to sixteen ounces 
of water (to stop continued action of toning), to be followed 
by a thorough washing in several changes of water before remov- 
ing the prints to the fixing-bath. 

B. Fixing Photographic Prints. 

After the washing following the saline bath the prints are 
immersed for fifteen to twenty minutes in the fixing-bath, keep- 
ing the prints in motion, the same as described for the toning 
process. A good plain fixing-bath may be made up as follows: 



382 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

Metric Weight. Apothecaries' Weight. 

180 grammes Hyposulphite of soda 6 oz. 

75 grammes Alum (powdered cr>'stals) 2.5 oz. 

II grammes Sulphite of soda (powered crystals). 3 drachms 

2000 c.c Distilled water 70 oz. 

When all these ingredients have been dissolved, add to the 
solution 25 grammes (6 drachms) borax, dissolved in 300 c.c. 
(7.0 oz.) hot water. 

This fixing-bath keeps indefinitely and may be made up 
in large quantity. It should be prepared fully a day before use. 

Directly after fixing, the picture should be washed, for one 
hour in clear running water or in ten to fifteen changes of water, 
at intervals of fifteen minutes, using a large tray or tank and 
keeping the prints separated so the water may have full access 
to the film and leach out all unconverted salts. If there is a ten- 
dency toward blistering the first change of water may be made 
saline. 

To make prints flexible and to rob them of a tendency to 
roll up it is recommended to immerse them in the following 
solution for a minute or two after removal from the final wash- 
ing: 

Metric Weight. Apothecaries' Weight.. 

90 c.c Glycerine 3 oz. 

120 c.c Alcohol 4 oz. 

30 c.c Distilled water i oz. 

It is advisable to drain the print well, by drawing its back 
over the edge of the tray, to remove as much of the surplus liquid 
as possible, before placing the print between blotters to dry 
under a light pressure. 

C. Formulas for Plain Toning-baths. 

For producing deep-purple or bluish-black tones in the final 
picture the following plain toning-bath is recommended: 

Metric Weight. Apothecaries' Weight. 

65 milligrammes. . . Pure chloride of gold i grain 

5 grammes Sulphocyanide of ammonia 80 grains 

315 c.c Distilled water 11 oz. 



PHOTOGRAPHIC PRINTING. 383 

For toning gelatine or collodion prints with platinum in 
place of gold, the following bath may be given : 

Metric Weight, Apothecaries' Weight. 

65 milligrammes. . . Chloroplatinite of potassium i grain 

0.5 gramme. . . . Chloride of sodium (salt) 8 grains 

0.5 gramme Citric acid 8 grains 

115c. c Distilled water 4 oz. 



D. Combined Toning- and Fixing-baths. 

When a " combined toning- and fixing-bath " is used, the 
prints, after removal from the printing-frame, require no pre- 
vious washing before immersion. 

A one-solution toning- and fixing-bath may be made up as 
follows : 

Metric Weight. Apothecaries' Weight. 

65 milligrammes. . . Pure chloride of gold i grain 

145 c.c Distilled water S oz. 

30 grammes Hyposulphite of soda i oz. 

4 grammes Sulphocyanide of ammonium. . . . i drachm 

I gramme Acetate of lead 15 grains 

I gramme Nitrate of lead 15 grains 

This solution should be well shaken before use, and it is best 
prepared a day before wanted. 

The following two-solution combined bath keeps better (in 
separate bottles) than the one-solution bath: 

A. 

Metric Weight. Apothecaries' Weight. 

30 grammes Hyposulphite of soda i oz. 

24 grammes Alum (powdered crystals) 6 drachms 

8 grammes Sugar (granulated) 2 drachms 

300 c.c Distilled water (cold) 10 oz. 

After these chemicals have all been dissolved in the cold water, 
15 grammes (4 drachms) borax dissolved in 60 c.c. (2 oz.) hot 
water are added and the mixture is well shaken. After allow- 



384 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

ing it to stand for twelve hours the clear liquid may be siphoned 
into a bottle marked "Solution A" or " Fixing-solution." 

The stock solution for toning is made by dissolving chloride 
of gold and sugar of lead in water. 

B. 

Metric Weight. Apothecaries' Weight. 

65 milligrammes. ! . . Pure chloride of gold i grain 

0.5 gramme Acetate of lead 8 grains 

30 c.c Distilled water i oz. 

This " gold solution " should be well shaken, but not filtered, 
before use. 

For the " combined toning- and fixing-bath " mix in the pro- 
portion of one part " gold solution " (sol. B) to eight parts " fixing- 
solution " (solution A). 

Half an ounce of stock solution B (" gold solution ") mixed 
with four ounces of stock solution A (" fixing-solution ") will 
tone about one dozen 5X8 prints. 

The double salt " chloride of gold and sodium," which is 
a mixture of chloride of gold and chloride of sodium, can be 
more easily handled than the pure chloride of gold in making 
the toning-solution, since it contains no free acid. If this crys- 
tallizable double salt be used in place of the pure chloride of 
gold in preparing the " gold solution " of a toning-bath, double 
the quantity will be required of the amount given for the pure 
chloride of gold in the preceding formulas. 

After the desired tone has been attained for the prints in 
the combined bath they should be removed to a saline solution 
(i teaspoonful of salt to sixteen ounces water) and immersed 
five minutes, after which they are washed in ten to twelve changes 
of water at intervals of fifteen minutes. 

To insure thorough fixing the prints may be immersed for 
ten minutes in the fixing-bath previously given, or in the follow- 
ing one, immediately after the first change of water following 
the saline bath. 



PHOTOGRAPHIC PRINTING. 385 

Metric Weight. Apothecaries' Weight. 

240 grammes Hyposulphite of soda 8 oz. 

30 grammes Sulphite of soda (granulated, dry) . . i oz. 

3.5 c.c Sulphuric acid i drachm 

960 c.c Distilled water 32 oz. 



After removal from this bath the prints should be thoroughly 
washed in clear running water for at least one hour or in ten 
to fifteen changes of water as previously noted. If the prints 
be now immersed in the following bath for five minutes any 
remaining trace of " hypo " will be removed and the film will 
become hard when dried: 



Metric Weight. Apothecaries Weight. 

960 c.c Distilled water 32 oz. 

30 grammes Powdered alum i oz. 

30 grammes Powdered chloride of sodium (salt) . i oz. 



After removal from this " hardener and short stop " the 
prints are again washed and dried. 

Combined baths should be used but once and they should be 
kept at a rather low temperature, not much over 50° F.; if the 
temperature is allowed to rise much above this the prints will 
become stained yellow and the darker tones will be tinged with a 
greenish tint. Prints should not be retained in the water over 
two hours. All trays should be kept scrupulously clean and 
not interchanged. A/Vhenever prints come out " splotchy " it is 
recommended to clean the trays, swabbing them out with diluted 
sulphuric acid. 

Those who prefer to " cut " their own gold for the " toning- 
solution " can make up a stock solution of the " gold solution B " 
in the above combined bath as follows: 



Metric Weight. Apothecaries' Weight. 

156 centigrammes . . Pure metallic gold 24 grains 

3.5 c.c Nitric acid i drachm 

10.5 c.c Muriatic acid 3 drachms 



386 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

After the gold has been fully dissolved, add 1440 c.c. (48 
ounces) distilled water and then add enough bicarbonate of soda 
to leave the solution slightly acid, just enough to turn blue litmus 
paper red. After shaking well filter into a bottle, add 25 grammes 
(384 grains) acetate of lead, and label this stock solution " Gold 
solution," or " Solution B " of the combined bath. 



CHAPTER XII. 

CONCLUSION AND REMARKS ON THE PRECISION OF THE 
"POLAR-ICONOMETRIC" METHOD AND GENERAL RE- 
MARKS ON TELEPHOTOGRAPHY. 



I. General Remarks on Phototopography. 

The main disadvantage in connection with phototopography, 
resting principally in the great" consumption of time in the pro- 
duction of the maps in the office, promises soon to be overcome 
through the perfections that are being made in the stereoscopic 
methods and instruments. The plotting of from fifteen to thirty 
control points, by means of the " polar- iconometric " method 
(by the iatersections of at least three radials or horizontal direc- 
tions for each control point), including the plotting (or the " ori- 
entation ") of the necessary picture traces, together with the 
verification of the focal lengths of the photographs, may be regarded 
as a good day's work. 

The main advantage in phototopography, on the other hand, 
rests in the rapidity with which the field work may be done. 
The phototopograph^r, spending niOsfof his time in traversing 
the country, stopping only long enough at the stations to photo- 
graph the panorama, to make sketches, and to observe a few 
sets of angles with the transit, can in a few good days cover a 
larger territory than is possible with any other surveying method. 

A phototopographic party is essentially an economic one, 
inasmuch as it comprises but one topographer, assisted by as 
many packers or hands as may be needed to transport the party 
outfit over the region that is to be surveyed. The time-con- 

387 



388 PHOTOTOPOGRAFHIC METHODS AND INSTRUMENTS. 

suming part of the work (the iconometric plotting) is independent 
of weather conditions and may be accomplished at any time 
by one or two iconometric draughtsmen in the office. 

The ready identification of points on the photographs is a 
matter of practice and will be found far less difficult than would 
appear at a first attempt. All apparent difficulties in this respect 
may soon be overcome by a comparative study of several pic- 
tures held side by side, and also by making use .of the numerous 
tests and constuctions that are available for this purpose, the 
most important of which having been given under Prof. G. Hauck's 
method. We have also seen that this difficulty disappears alto- 
gether when applying the stereophotographic methods. 

To economize in time, the general progress of the field work 
should, as far as possible, be regulated by the weather and cli- 
matic conditions of the region to be surveyed. Elevated sta- 
tions should be occupied during good weather, as the lower sta- 
tions, being more readily accessible and less often obscured by 
clouds, may be successfully occupied at almost any time. Good 
work may often be done at the lower stations when work on 
the mountain peaks i^'iknpossible, owing to misty weather, snow, 
or strong winds prevailing here, while the lower altitudes may 
be free from either during the same time period. 

Special attention should be given to a good selection of the 
camera stations, with reference to the elevations and the dis- 
tances of the terrene points that are to be determined, to the 
focal length of the camera, the desirable degree of accu- 
racy, the scale of the map, and the general character of the 
country. A diversified and broken terrene will require more 
stations to obtain a good topographic development and repre- 
sentation on the map than a more regular section; the camera 
stations, however, should be selected to obtain a full control 
of all depressions, valleys, and general topographic features 
from the smallest number of camera stations. Every feature 
that is to find a rcfresentation on the map should have been 
photographed from at least two, better three, stations. If a 



GENERAL REMARKS ON PHOTOTOPOGRAPHY. 389 

part of the terrene be visible from two stations only its icono- 
metric location in horizontal plan may be accepted if its con- 
trol points on the plan have been determined by good intersec- 
tions (if the horizontal lines of direction intersect each other 
at angles of 40 to 90 degrees), otherwise " vertical " intersections 
or other means for checking' the location of these points will 
have to be adopted unless additional stations may be occupied, 
while the party is still in the field, to obtain lines for a third inter- 
section. 

On the other hand, to reduce the number of photographic 
plates that are to be transported, to simplify the iconometric 
office work, and with due regard to the limited length of the 
working-season in mountainous regions, it will be advisable 
not to occupy more stations than are actually required for the 
proper development of the terrene. 

To secure the proper control for the location of the camera 
stations on the map, at least three, better four, lines of direc- 
tion to surrounding geodetic (triangulation) points should be 
observed from each camera station. If that many triangula- 
tion points be not visible from the station, that number of direc- 
tions should be observed anyway, pointing on other well-defined 
points (to supply the deficiency in triangulation points) that 
may have been located before (as other camera stations), or 
which may be located by later observations to be made at sta- 
tions still to be occupied. Every station should be marked 
with a signal before leaving, and such signal is to be observed 
upon from stations subsequently occupied, observing both hori- 
zontal and vertical angles. 

Regarding the selection of the hours that are most favorable 
for photographing the panorama views, one should be guided 
principally by local conditions. Generally speaking, views of 
identical regions should, if possible, be taken at the same time 
of day and under similar atmospheric conditions, to facilitate 
ilie recognition of identical points on the different views; the 
actual shadows will then be alike in the different pictures. Pho- 



390 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

tographing slopes altogether in shadow and exposing plates when 
the sun is low should be avoided as far as possible. In the latter 
case additional trouble may arise from the fact that one or more 
pictures taken in the direction toward the sun may be affected 
by halation; they will at best be more or less flat and will always 
be deficient in details. Still, the phototopographer is seldom 
privileged to select the most favorable time for making the expos- 
ures, being governed by many considerations, having but a limited 
time at his disposal, having to contend with moving cloud-masses, 
inaccessibility of points, etc. Sometimes views will have to be 
taken toward the sun if they are not to be dispensed with alto- 
gether, but with the exercise of care and judgment photographs 
may be obtained that will be of value for the iconometric plotting, 
even when taken under such adverse conditions. The camera- 
lens, however, should always be carefully shaded when taking 
pictures under such untoward conditions. 

It may generally be stated that the best results in mountainous 
countries are obtained when the plates are exposed in the latter 
part of the forenoon, the elevated peaks being mostly " hooded " 
in clouds during the afternoon. Although these clouds fre- 
quently disappear again late in the afternoon or toward evening, 
still at this late hour all details of the valleys are obscured if 
not perfectly hidden in a misty darkness. 

When everything is favorable, the entire work at a camera 
station may be finished within an hour and a half, or two hours 
at the longest, and as three well-placed stations will control the 
horizontal and vertical representation of an extended area, a 
large territory may be reconnoitered phototopographically in a 
comparatively short time. 

The time consumption for accomplishing the field work of a 
detailed phototopographic survey will be about the same as 
for a more generalized survey, as about the same number of 
photographs will be required in both cases. The difference, 
however, appears at once during the execution of the office, work. 
In the first case the number of points to be plotted iconometrically 



PRECISION OF THE POLAR-ICONOMETRIC METHOD. 391 

may be very large, while in the latter case it will naturally be 
very small, comprising points which characterize and control 
the main features and forms of the terrene only. 

We find, therefore, that outside of topographic reconnaissance 
surveys in moimtainous districts the phototopographic methods 
are particularly well adapted for executing topographic pre- 
liminary surveys made for that class of engineering works in 
which the final and best location of the enterprise depends upon a 
comparative study of the different sites as represented on the 
topographic maps. Only a limited number of "points would 
have to be determined iconometrically to reach a decision whether 
the site under consideration fulfills the required conditions. After 
the best site has been determined upon, a more detailed and 
accurate map may be constructed from the same field data 
without having to supplement the original survey, either by 
additional observations or photographs, every panorama view 
giving the means to plot therefrom (iconometrically) almost an 
unlimited number of terrene points. 



n. Precision of the " Polar-iconometric " Method. 

The desired degree of accuracy in a survey will generally 
determine the class of instruments and the methods to be used 
in its execution. To ascribe, therefore, the various surveying- 
cameras and phototheodolites their proper places among survey- 
ing-instruments, it will be of importance to know, or to ascertain, 
what degree of precision may be obtainable with each repre- 
sentative type of a special class. This has been done for some 
of the special types that have been described in the preceding 
chapters, and we will here enter upon a more general considera- 
tion of the precision attainable in the so-called "polar" or- 
" radial " method of iconometric plotting. 

We have seen that the graphic methods of phototopography 



392 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

3,re very similar to those of the plane table. It is generally 
accepted that azimuthal errors in the directions of the " radials," 
drawn with the plane-table alidade on the plane-table sheet, 
' should not exceed' 1.5 minutes in arc, and we shall see that when 
the principal focal length of the camera does not fall below 1 50 mm. 
this same degree of accuracy in the angular values of the lines of 
direction may be obtained iconometrically from the panorama 
views. 

The plotting from photographic perspectives being dependent 
on the measurements of coordinates made directly on the pho- 
tographic perspectives, the attainable degree of accuracy will 
greatly depend both upon a good definition and upon the mathe- 
matically correct representation in perspective of the landscape 
upon the flat field of the perspective (negative). 

The precision in the mechanical determination of the coor- 
dinates of any point pictured in the photographic perspective 
depends not only upon the more or less good definition of the pic- 
tured point, but also upon the means used for measuring these 
coordinates. According to Dr. Meydenbaur, the definition of a 
photograph obtained with a suitable lens will be sufficiently 
good for phototopographic purposes if a point, or rather its 
" phase," or the circle of diffused light that represents the point on 
the picture, does not exceed o.i mm. in diameter; hence all pic- 
tured lengths should be obtainable within a limit of dx = dy~o.i 
mm. Unless special measuring devices are employed, this 
value will also represent the attainable degree of accuracy in 
making direct measurements on clear and well-defined nega- 
tives with ordinary drawing instruments (dividers and trans- 
verse scale). It is evident from the foregoing that a computa- 
tion carried out to several places of decimals (analytic method) 
cannot increase the accuracy of the resulting map as long as the 
elements of the perspectives upon which such computations 
are based have been obtained with a degree of accuracy not 
closer than o.i mm. All iconometric plotting being dependent 
on direct measurements executed on the photographic plates^ 



PRECISION OF THE " POLAR-ICONOMETRIC METHOD. 393 

the transcription of the pictured data into the horizontal pro- 
jection plane is best done graphically. 

Numerous experiments have shown that 0.14 mm. is the 
smallest discernible difference in length that the average eye 
may distinguish without optical aids. With a beveled scale 
graduated to 0.5 mm. a fairly well-trained eye can determine 
lengths correctly within o.i mm., while a well- trained eye reaches 
the limit at 0.06 mm. By the use of special scales fitted with 
verniers and microscopes the attainable degree of accuracy in 
the measured lengths may be increased to reach 0.03 mm. 

The attainable degree of precision in the angles may be found 
from the equations 



X 

tan oc=j: (for horizontal angles), 



y 

tanj8=— 7 (for vertical angles), 



where 31;= abscissa of the pictured point; 
y= ordinate of the pictured point; 
Z)= distance line of the perspective. 

By differentiation we find 

dy ^-y 

d tan B=—p= — . — =dx. 

VD^+x^ VD^ +x^ 

If we express x by an aliquot part of D, x=—, and disregard 
= -s and introduce the arcs in ulace of the tangents of the 



394 PHOTOTOPOGEAPHIC METHODS AND INSTRUMENTS. 

angles, we will find 

^ ^ dx 
da (in seconds of arc) =206265^; 

dy 
dp (in seconds of arc) =206265-^. 

These equations show that the angular errors are directly 
proportional to the degree of accuracy attainable in the measured 
lengths (on the photographs) and indirectly proportional to the 
focal lengths of the lenses that are used. Assuming the attaina- 
ble degree of accuracy in the measured lengths to be dx=dy =0.1 
mm., we will find the attainable degree of accuracy for the 
angles for the following five focal lengths to be : 

For a focal length = 20 cm. the angular accuracy is i' 43" 
II << II _„- II << " << " i' 21;" 

<< £( 11 ^20 " " " " " l' 09" 

«« (I << __- <( n " " " o' Kq" 

" " " =40 " " " " " o' 52" 

The longer the focal length of the lens the smaller the angu- 
lar error will be, and although it is desirable to have the photo- 
graphic details as large as possible for a better identification 
of the terrene points on different photographs and to increase 
the attainable degree of accuracy, still, to reduce the weight of 
the instrument as much as possible the focal length will natu- 
rally be circumscribed for instruments to be used in mountainous 
regions, where portability and compactness are among the prime 
factors to be considered in their construction. 

The attainable degree of accuracy for any particular camera 
may be ascertained experimentally, after the methods of Dr. W. 
Jordan and Capt. E. Deville, by observing a series of horizontal 
and vertical angles, included between lines of directions, to a 
series of well-defined points of known positions and elevations, 



PRECISION OF THE " POLAR-ICONOMETRIC " METHOD. 395 

taking also a photograph of the same points (in vertical plane) 
from the same station and from the same elevation. The hori- 
zontal optical axis of the camera should have the same height 
as the horizontal optical axis of the transit telescope when the 
angles were measured, or the difference in the elevation between 
both should be taken into account. 

The focal length of the photographic perspective and the 
correct positions of its horizon line and principal point may now 
be determined from the requisite number of observed directions 
to the known and plotted points. The remaining angles, meas- 
ured in excess of the required number just referred to, may 
well be used for comparison with the corresponding values, 
obtained iconometrically from the oriented picture trace, to 
arrive at a knowledge of the attainable degree of accuracy of 
the camera in question. 

If the camera-lens was of good quality (for surveying pur- 
poses), if the selected points were well defined (both in nature 
and on the negative), if the sensitized surface of the plate con- 
tained no gross inequalities or irregularities, and finally, if the 
measurements of the coordinates were carefully made on the 
negative, the lengths obtained iconometrically on the plotting- 
sheet, compared with those obtained trigonometrically, should 
differ by no more than o.i mm. in actual length, and the icono- 
metric angles should differ from those that were observed by 
no more than from i to '2 minutes in arc for objectives with 
focal lengths from 350 to 150 mm. and commanding a hori- 
zontal field of view from 50 to 60 degrees. 

This degree of precision may be increased by a reduction 
in the field of view of the lens by using plate glass for the nega- 
tives and by making microscopical measurements of the coor- 
dinates of the pictured points. Still, when photography is applied 
in this way, including precise computations (analytic method), 
for surveying purposes, one of its main advantages is lost sight 
of and sacrificed, since one of the chief advantages in photo- 
topography rests in the numerous and varied constructions (based 



396 PHOTbTOPOGRAPHIC METHODS AND INSTRUMENTS. 

on the laws of perspective) that are available in transferring 
the photographically recorded data to the plotting-sheet. 

The attainable degree of accuracy in the elevations obtained 
iconometrically may also be ascertained in the manner indicated, 
but the results will only hold good for points having the same 
distances as those observed upon with the transit from the camera 
station. 

To find the limit in the distances that are to be included 
for a certain lens when a given degree of accuracy in the ele- 
vations is to be maintained, we may proceed in the following 
manner: 

We have the known relation 



III 



or 



a a-f a—f 

where'a=distance between object and second nodal point of 
the lens; 
&= distance between image and second nodal plane of 

the lens; 
/ = constant focal length of the camera-lens. 
We had seen that the elevation (5) of any pictured point 
above the horizon line stands in the same relation to the ele- 
tion (A) of the point itself, above the horizontal plane that passes 
through the horizon line, as the horizontal distance (&) between 
lens and image is to the horizontal distance (a) between the 
lens and the horizontal projection of the point itself. 

B:A=b:a, 
or 

A a-f 



PRECISION OF THE " POLAR-ICONOMETRIC ' METHOD. 397 

In topographic surveys the distance a will be so great compared 
with / that the latter may be neglected in comparison with a, 
and we will have, with close approximation, 

j = -, or a (approx. ) = ^ • 

Hence, whenever the considered distances a are large com- 
pared with the focal length, we may place b=f, which means 
that the phototopographic cameras may be constructed with 
constant focal lengths, which in point of fact is the general 
practice. 

If differences in the elevations of points of the terrene are 
to be deduced photogrammetrically within a limit of error not 
exceeding one meter, the pictured length (B) of a meter in nature 
(A) should not appear shorter than o.i mm., and we find for 
the following four typical values of focal lengths the correspond- 
ing values for a, representing the extreme distance Hmit between 
the camera station and an object one meter high that is to have 
a pictured height of at least o.i mm. as follows: 

a= 750 m. 1000 m. 2000 m. 2500 m. 
for /= 75 mm. 100 mm. 200 mm. 250 mm. 

It would require the exercise of special care and the measure- 
ments of the coordinates would have to be made directly on 
the negatives if this limit of error is to be maintained. Care- 
fully made contact prints permit measurements to be taken 
(with the ordinary instruments of the draughtsman) correctly, 
with an average error of not less than 0.25 mm., and taking 
this limit as commensurate with good work, we will now have: 

a= 300 m. 400 m. 800 m. 1000 m. 
for ,/= 75 mm. 100 mm. 200 mm. 250 mm. 

These values clearly demonstrate the necessity for a close 
disDosition of the camera stations over the area to be surveyed 



398 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

(from 600 to 2000 m. apart) for detailed work, thus materially 
circumscribing the advantages of phototopography, practically 
excluding its application to the topographic surveys of rugged 
mountains, a terrene for which the phototopographic methods 
are best suited and to which, as a matter of fact, they have pri- 
marily been applied with the greatest success as long as larger 
errors (exceeding one meter) in the elevations were permissible. 
Thus, if the error in the iconometrically determined elevations 
may attain twenty meters, and if the coordinate measures may 
be correctly obtained on the prints within an error of 0.25 mm., 
we will find the following values for the effective range for the 
same four focal lengths to be: 



0=6000 in. 8000 m. 16000 m. 20000 m. 
for /=. 75 mm. 100 mm. 200 mm. 250 mm. 



For a permissible error in the elevations of twenty meters 
the camera stations may be located at intervals of from 10,000 
to 40,000 meters, for cameras with focal lengths from 75 to 250 
mm., provided the character of the terrene does not require a 
closer disposition of the stations to obtain a better development 
of the intervening terrene forms. 

Attempts have been made to increase the effective range of 
the surveying-cameras by constructing them with variable or 
adjustable focal lengths; this, however, produces complications 
and opens additional sources of error. Better success in this 
direction has been obtained indirectly by using photographic 
enlargements (" optical projections ") of the original negatives 
in the iconometric plotting. This method is successfully pursued 
in Canada under Capt. E. Deville, Surveyor-general of Dominion 
Lands, who advises the use of enlarged positives on glass for work 
requiring a high degree of accuracy. 

If the reconnaissance of a given terrene has established the 
greatest limit between the camera stations to be 2a, we can, 



PRECISION OF THE " POLAR-ICONOMETRIC METHOD. 399 

for a required degree of accuracy, find the proper focal length 
of the camera from the equation 

The accuracy attainable in iconometric plotting depends 
greatly upon the distances between the several camera stations. 
We had seen that the precision of a plotted line of direction 
depends upon the accuracy with which the abscissa x may 
be transferred from the photographic perspective to the plotted 
picture trace. The camera station being fixed, the accuracy 
in the direction of the ray will depend solely upon the value 
or the amount of the error dx with which the measured abscissa 
X may be affected. With a given limit dx for a constant 
focal length /, the error in position of a plotted point will increase 
with the distance of the latter from the camera station. All 
plotted points falling between the picture trace and the plotted 
camera station are affected by no errors larger than dx, and if 
the permissible limit of error in the map is not to exceed dx, 
we will have to select the camera stations sufficiently close together 
that no points are determined iconometriCally which fall beyond 
the traces of the pictures from stations whence the, latter were 
obtained. The base-line lengths are plotted to scale, while the 
constant focal length of the camera enters into the iconometric 
construction in its original length; hence the reduced lengths 
separating successive camera stations on the plan should not 
surpass the true focal length /. For a scale of map of i/n 
the largest base line should be ^f-n, measured in millimeters. 
For a focal length of / = 2oo mm. and a scale of map of 1/20000 
the base line should not exceed 200X20000 mm. or 4000 meters, 
measured in the plotting-scale. 

All elevati&ns determined from the photographs should be 
corrected for curvature and refraction. Refraction apparently 
affects directions more in the vertical than in the horizontal 



400 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

sense. Prof. S. Finsterwalder found in his phototopographic 
surveys made in 1888 and 1889 that the accuracy in the ele- 
vations increases directly with the distance of the observed point. 
The elevation of a point 500 m. or less distant from the camera 
station was three times less accurately obtainable, iconometric- 
ally, than the elevations of points between 2500 to 5000 m. dis- 
tant. 

Generally speaking, terrene points determined iconometrically 
will not be provided with signals, and the identification of a well- 
defined point on several photographs may be affected by an 
error of from one to two minutes in arc. Even points that may 
have been supplied with signal poles of the ordinary size and 
length, when five hundred meters and more distant from the 
camera stations, will appear on the photographs as having no 
signals, yet when viewed through the telescope of the ordinary 
field transit, the same poles may appear very clearly and well 
defined, even at distances up to several kilometers. Artificial 
signals will, therefore, be of little use in general iconometric 
plotting; still, as well-defined points are a necessity to insure 
good results, the camera stations should be located not too far 
apart, and thf selected reference points of the pictures should 
not only be sharply defined, but the instrumental measurements 
to the same objects (to provide the needed data for the orienta- 
tion of the picture traces and for the control) should be made 
as nearly as possible at the time of the exposure of the plates, 
that such points may be seen under the same conditions of illu- 
mination that prevailed when they were photographed. 

Regarding the expeditiousness of the phototopogr^hic 
methods considered in the preceding chapters, it may be stared, 
from the experience of Dr. S. Finsterwalder, that so-called 
topographic surveys of mountain regions, for which an artistic 
representation of the terrene, in conformity to its natural appear- 
ance, may be claimed rather than accuracy, may be made by 
an expert plane-tabler, combining liberal sketching with the 
instrumental survey, on 1/25000 scale in less time than it would 



PRECISION OF THE " POLAR- ICOXOilETRIC METHOD. 4OI 

take the phototopographer to select, locate, and occupy the 
camera stations required for an " accurate " phototopographic 
survey to be plotted on i/ioooo scale. This holds good for 
surveys in mountains of an Alpine character arjd sparse vegetation. 

Besides the errors considered in the preceding paragraphs, 
there still remains anotlier source of error to be considered in 
the photographic developing and fixing process of both the nega- 
tives and their positives. Distortion in the sensitized gelatine 
coating of the modem dr\--plate during the process of develop- 
ment is rarely perceptible if the work is carefully done to avoid 
so-caUed " frilling " of the film. The mean value of such dis- 
tortion, according to Dr. H. C. Vogel, amounts to o.oi per centum 
of tlie length, and as the plates used in phototopography are 
never large, the errors due to this cause may be disregarded 
altogether. 

The distortion in the positives (particularly if made on paper 
that requires subsequent development), however, is mostlv so 
large that it must be considered when using such prints 
iconometricaUy. This distortion, moreover, is irregular, being 
smaller in the direction with the grain or fibers of the paper, 
where it piay amount to 0.5 per centum of the length, and larger 
in the direction across the fibers, where it wiU amount to about 
I per centum. The constants of the camera, therefore, and 
the coordinates of the principal points of control should pref- 
erably be obtained from the negatives. For the iconometric 
plotting of the topographic detaUs enlarged projections on bromide 
paper may be used. Capt. DeviUe has recently substituted a 
hea\Tly coated " platino-bromide " paper for the ordinar}' silver 
bromide paper heretofore in use. The length of exposure for 
the enlargement is made directly dependent upon the density 
of the original negative. 

To give ready means for controlling or correcting the dis- 
tortion affecting the paper prints nearly all modem surveying- 
cameras are provided with a metal frame permanently fixed in 
the image plane of the lens with constant focal length, the inner 



402 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

margins of the frame having a graduation that is photographically 
reproduced on the outer margins of the negative; thus the amount 
of distortion that may affect the positive can readily be ascer- 
tained in the directions of both the horizon and the principal 
line of the photographic perspective. " Backing " the paper 
prints would open another source of error even greater than 
those just referred to. Dr. Stolze observed a permanent expan- 
sion of five per centum for prints that had been mounted vi^hile 
in a damp condition. 

III. General Remarks on Telephotography or Long-distance 

Photography. 

The range in the field of application of photography to sur- 
veying and military reconnaissance has been considerably enlarged 
during the past few years by the invention of the telephoto- 
lens combination, used for obtaining well-defined photographic 
views of objects at long distances, sacrificing or reducing the 
angular value that the plain lens commands for the sake of 
enlargement of the view, producing thereby the same effect as 
if the view had been taken from a point of view much nearer 
to the distant object. 

Long-distance photography (" telephotography ") was proba- 
bly first studied in France, principally by Matthieu and Lacombe, 
and more recently by Guillemont and Jarret. This subject 
continues to receive much attention in France, particularly among 
the army officers stationed at Grenoble, as has been mentioned 
in Chapter I. Quite recently telephoto instruments have been 
devised and placed upon the general market by Hondaide 
and Derogy in Paris. The lunette d'Etat-Major, one of the 
smaller types of telephoto instruments, manufactured by Arizard 
in Paris, controls distances up to 5 km. and weighs only about 
8 kg. 

The Intelligence Office of the British War Department is 
also doing a great deal towards promoting the efficiency of the 



GENERAL REMARKS ON TELEPHOTOGRAPHY. 403 

telephoto instruments and towards familiarizing British officers 
with the telephotographic reconnaissance methods. The inven- 
tion of the telephotographic lens combination is ascribed by 
Th. R. DaUmeyer to Peter Barlow, who combined a negative 
lens with the astronomical telescope as early as 1834. 

Researches in long-distance photography have notably been 
made by T. R. Dallmeyer, London; Dr. A. Miethe, Potsdam; 
Dr. Steinheil, Munich; Prof. R. Spitaler, Vienna, and others. 

One of the main defects in phototopography rests in the small 
scale to which the distant landscape features are reduced on 
the negative, requiring precise and minute measurements to be 
made on the negative in connection with the iconometric con- 
structions. This defect is primarily conditioned by reducing 
the weight of the surveying-cameras to a minimum. Cameras 
with constant focal lengths are principally in use for topographic 
surveys in mountainous regions, where the reduction in weight, 
as previously stated, means a great deal toward ultimate success. 
For this class of work the use of objectives of long focal 
lengths is precluded and we find topographic-surveying cameras 
supplied ^vith lenses having constant focal lengths from 75 to 
350 mm. 

In the preceding chapter the effective ranges of four-lens 
types have been discussed, fully demonstrating the desirability 
of providing means for obtaining special perspective views of 
terrene sections lying beyond the reach of the ordinary camera- 
lens, for certain inaccessible localities and particularly for mili- 
tary reconnoitering purposes. 

By adding the so-called telephoto attachment to the original 
camera-lens an enlarged image of the view is photographed 
directly on the plate (in the field). .The use of the telephoto 
attachment (it may easily be removed) has the advantage that 
the selection of the distant views rests entirely with the topog- 
rapher in the field, as he can best decide whether by taking 
such a telephotographic view from one of the ordinary camera 
stations a lengthy trip of the party in that direction may be saved, 



404 PHOTOTOPOGRAPHIC- METHODS AND INSTRUMENTS. 

or whether a special advantage may be derived from a series 
of such views taken during the occupation of some prominent 
or isolated peak. In short, the phototopographer can best tell 
whether time may be saved by supplementing the ordinary pano- 
rama views with some special long-distance views. 

In high altitudes mists and clouds frequently hide the higher 
peaks from view for weeks at a time, and it may often save many 
days of waiting if the phototopographer be provided with a 
telephoto attachment for his camera-lens, to enable him to photo- 
graph distant terrene features that may casually be visible on 
a clear day while taking the panorama views for the develop- 
ment of the topography in the immediate neighborhood of the 
station. 

The topographer may not see the same peaks free from " cloud- 
hoods " again during the rest of the season, at least not from 
that particular direction. It will seldom require more than one 
such telephoto view in several panorama sets, the critical points 
pictured on the other plates being near enough to the station 
not to need special enlarging. 

Little regarding the telephotographic results, obtained prin- 
cipally under military auspices, reached the general public 
until Dr. A. Miethe, in Germany, and T. R. Dallmeyer, in Eng- 
land, each apparently independent of the other, published de- 
scriptions of their telephoto-lens combinations. The principal 
difference between their combinations seems to be that Dall- 
meyer uses a " portrait-lens " in connection with the " nega- 
tive-lens combination," while Dr. Miethe combines a photo- 
graphic lens of the " rapid landscape type " (Steinheil's) with 
the negative combination. The construction of both these 
telephoto objectives rests upon the same principles, and their 
combination is composed of two biconvex lenses interposed 
between the camera-lens and the sensitive plate. By changing 
the distance between the biconvex lenses more or less enlarged 
images will be photographed on the plate. These enlarge- 
ments are made at a sacrifice of the field of view commanded 



GENERAL REMARKS ON TELEPHOTOGRAPHY. 405 

by the camera-lens alone, and it would require a large number 
of plates to cover the entire horizon with telephoto views; still, 
in phototopography the general panorama views wUl be taken 
with the simple camera-lens, adding the telephoto attachment 
only for special views of distant " heads," or saddles of valleys, 
inacessible mountain peaks, etc. 

Since the " negative element " of the telephoto combination 
lens produces a picture of the distant view in the image plane 
of the camera with a sharp definition and a richness in detail 
far surpassing the perceptive power of the eye, and, on the other 
hand, the wide angle type of camera-lens (the " positive ele- 
ment ") reproduces, with an evenly good definition, views sub- 
tending angles far surpassing the field of view of the eye, it will 
be plain that the combination of these two elements into one 
optical system will give results that cannot be obtained in any 
other manner. The telephoto combination works particularly 
well for picturing objects that are about equidistant from the 
lens (for picturing objects that are in the same frontal plane), 
but when objects are photographed that are at different dis- 
tances from the instrument the image will be more or less dis- 
torted by the effects of spherical aberration. This spherical 
aberration may be reduced in a measure by stopping down 
the camera-lens; this, of course, decreases the intensity of the 
ill umin ation of the image, reduces the rapidity of both the lens 
and the plate, contracts the already small field subtended by 
the telephoto combination, and consequently necessitates an 
increase in the time of exposure, with the incident risk of com- 
municating tremors to the camera which would be detrimental 
to the definition of details in the telephoto-plate. 

Dallmeyer recommends, therefore, for a " really useful tele- 
photographic lens system" the combination of the rapid recti- 

IF F\ 
linear type of lens (^ or - ), together with a negative lens of 

half its focus. In this proportion the latter may be made of 
larger diameter than the lenses of the positive element, thereby 



4o6 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS. 

increasing both the included angle and the illumination of the 
plate, as when using a negative lens of smaller diameter than 
that of the positive element. With observance of this recom- 
mended proportion of the foci the negative element (attached 
to the positive lens) will not become inconveniently large and 
heavy. Negative lenses with foci a little longer than half the 
focus of the positive set may safely be used to increase the field 
of view (the included angle) at a reduction in the magnification 
of the image. 

When using a telephoto-lens the instrument should be well 
and rigidly supported, as the smallest tremors of the camera 
become magnified in their effect upon the image and probably 
would spoil the plate for phototopographic purposes. The 
focusing should be very carefully made, using the same stop 
that is to be used for the subsequent exposure. Generally speak- 
ing, the best results will be obtained on a calm day after a rain 
or when the atmosphere is perfectly free from smoke and dust. 
To obtain good results for distant mountain views, of course 
a yellow color-screen or ray-filter will have to be provided. It 
is also of great importance that the photographic plate be in 
perfect contact with the metal frame of the camera, as even a 
slight irregularity in this respect would spoil the negative for 
iconometric purposes. 

Telephotographic cameras combined with phototopographic 
methods no doubt will play an important part in modern wars 
and maneuvers where smokeless powder will be in general use, 
to reveal the positions of hostile troops and enable the observing 
officers to obtain a correct idea of the positions of the enemy, 
and consequently of his plans (inasmuch as these may be 
deduced from the recognized disposition of the forces in the 
field). Light transportable " conning towers " supplied with 
" telescoping " or extension tubes and prism reflectors will 
probably form the most satisfactory support for the telephoto- 
lens combination when used for strategic reconnoitering pur- 
poses. For use in the army a telescoping-tube system could 



GENERAL REMARKS ON TELEPHOTOGRAPHY. 407 

be devised to serve in place of the center pole of the dark- 
tent. 

The following are some of the better-known publications on 
long-distance photography and telephoto-lens attachments: 

T. R. Dallmeyer. "The Telephotographic Lens," published by J. H. Dall- 

meyer. London, 1892. 
T. R. Dallmeyer, F.R.A.S., etc. "Telephotography: an Elementary 

Treatise on the Construction and Application of the Telephotographic 

Lens." London, 1899. 
M. le capitaine du G^nie Bouttreaux. "Mtooire sur la tdldphotographie." 

Nro. de Sept., 1897, Revue du Gdnie. 
Mtooire du chef de bataillon Allotte de la Fuye, commandant I'Ecole du 

Genie de Grenoble. "Sur I'Emploi des appareils photographiques pour 
. les observations k grande et a petite distances." Autographic k I'Ecole 

du Genie de Grenoble, 1891. 
Aug. VAUTiER-DirFOtiR. "Sur la TClCphotographie." Bulletii; de la 

SocidtC Vaudoise des Sciences naturelles. No. 143. Lausanne, 1902. 
NicoDEMO Jadanza. "II Teleobbietivo e la sua Storia.'' Torino, Carlo 

Clausen, 1899, Estratto dalle Memorie deUa Academia delle Scienze di 

Torino, Serie II, T. XLIX. 
Max Loehr, Chef de la maison C. A. Steinheil, Paris. "Du Tdlfobjectif." 

Annexe du bulletin de la Sociftd franjaise de Photographie, 1902. 
Max Loehr. "Sur la Determination des M&ures du T61&bjectif." Bulle- 
tin de la Sociftd franyaise de Photographie, 1902. 
Dr. P. Rudolph. "Anleitung zum Gebrauch des Teleobjectifs von der 

Firma Carl Zeiss.'' Jena, 1896. 
C. A. Steinheil. "Ueber Fernphotographie." Phot. Correspondenz, 

1892. 
Dr. A. Miethe. "Photographische Optik." 
Lechner's Mittheilungen, Feb. 1892. 
Mittheilungen aus dem Gebiete des Seewesens, 1894. 
Revue du Cercle Militaire, 1895. 
The American Annual of Photography, 1896. 
Zeitschrift fuer Vermessungswesen, 1892. 
Anthony's Photographic Bulletin, 1892 to 1894, etc 



INDEX. 



A. 

PAGE 

Abbe, Prof. j^g 

Aberration lyg 

archomatic 16, 

chromatic igo 

spberical 16, 

Abruzzi Mountains 24 

Academy of Sciences, Madrid 28 

Paris 7 

Academy, Royal Building, Berlin i^ 

U. S. Military, West Foint 4 

Swedish Military 22 

Accampamento Reale gp^ 10,^ 106 

Accelerating development icy 

Accelerator 257, 361 

Acetate of lead ^go^ 084 

soda 3g3 

Acetic acid j^e 

Achromatic aberration j5, 

'ens 179, igi 

Acid, acetic '. ^^^ 

carbonic ,^ , 

"trie 357, 383 

hydrochloric 362 372 

muriatic 353, 385 

"I'tric 38s 

oxalic 352 

pyrogaUic ; 358 

sulphuric sss, 361, 367, 385 

sulphurous 366, 373 

409 



410 INDEX. 

PAGB 

Ackerblom, Ph 22, 23 

Actinic action of light 338 

power of light 346 

Aerial perspective 337 

Alaska, southeastern 197 

Alaskan boundary 31 

Commissioners 3i> 32, 135, 197 

Phototopographic Reconnaissance 3ij 13S 

Albumenized paper 378 

Alcohol 188, 362, 382 

Alidade, Danish plane-table 242 

Alidade Holomdtrique 260 

Alkaline solution 350, 351, 358 

Alophe 8 

Alpenstocks 212, 222 

Almeria 28 

Altazimuth 195 

Alum ; 357, 367, 382, 383, 385 

Aluminum camera-box 186, 189, 199, 201 

Amber powder 375 

Amidol 366 

Amidol developer 366 

Ammonia-bath 378 

Ammoniimi chloride 370 

Amrein, Prof 23 

Anastigmat lens 164, 222, 226, - 29, 239, 243, 247, 25^, 342 

Antiplanatic lens (Steinheil's) 212 

Antihalation plates 342 

Andreas, F. C 15 

Aperture 166, 167, 176, 177, 215 

Aplanatic lens (Steinheil's) 183, 222 

Apua 25 

Arago iv, 6 

Archer, Scott iv 

Architectural surveys vii 

Argentic chloride iii, iv, 372 

Arizard 402 

Artificial horizon 188, 193 

Assistant topographer 187, 193, 195, 196 

Axis, optical 202, 206 

perspective 65, 66 



INDEX. 41 1 

PAGE 

Azimutale fotografico 224, 227 

Azimuth camera 226 

compass 203, 223, 227, 266 

photographic determination of 224, 225 

B. 

Backing plates 342 

prints 402 

Balbreck 255 

Barrel-shape distortion 177 

Bart Glacier 24 

Base, stereophotogrammetric 323, 326 

Batut, Arthur 11 

Becker, Prof 20, 23 

Beautemps-Beaupr€ 5, 6, 223 

Bennati, L 27 

Benzine 375 

Benzole 375 

Bemhardi, Capt 12 

Berthaud, Col 11 

BerteUi, Giuseppe 27 

Bessel 61 

Bicarbonate of soda i . . 360, 386 

Bichloride of mercury 296, 370 

Bichromate of potassium 362, 372 

sodium 379 

Biconcave lens 165, 173, 174 

Biconvex lens 165. 168-173, ^79 

Bildmesskunst 13, 47 

Binocular microscope 318, 321 

telescope 309 

Blistering 378, 382 

Bisulphide of sodium ,. 365 

Blue haze 337 

Bluemke, Dr. A 15 

Board, photograph 137, 138, 148 

Bock, Major 16, 17 

Boettcher, Prof 368, 372 

Bohnenberger • 61 

Borax 379. 3^2 



412 INDEX. 

PAGE 

Bornecque, J ii 

Bouttr&u, Capt 1 1 

Brasset Frferes 253 

Bridges-Lee 23> 262^269 

Brit sh Columbia boundary 31 

Broken telescope 260 

Bromide of potassium iv, 355, 356, 359, 361, 364, 365, 371 

silver v 

paper 365, 378 

Bromo-hydrochinon developer 364 

Brown stains 373 

Brunner 7, 28 

Buc 28 

Bunsen ■ 346 

Buonome, Giacome 27 

Bureau, Swiss Topographic 23 

Canadian Topographic 135 

Bush, E 128, 183 

C. 

Calotype iv 

Camera-box, aluminum 186, 199 

Camera clara. v, 6 

crosswires 202, 213, 265, 266 

diaphragms 186, 205, 259 342 

frame graduation 233 

levels 184-192, 197, 200, 203, 205, 233 

obscura 6^ 42 

plates, size of (Bridges-Lee) 268 

(Ney) 253 

(Vallot Frferes) 255 

(von Huebl) 253 

stations, distances between ,„„ 

select'on of ,88 

surveying 182 

(Bridges-Lee's) 262-269 

(Deville's) 186-196 

(Doergens') 184 

(Finsterwalder's) 227-230 

(Haflferl's) 242-243 



INDEX. 413 

PAGE 

Cameia surveying (Italian) 211, 219-227 

(Koppe's) 234-241 

(Laussedat's) 244 

(Le Bon's) 184 

(Lechner's) 184 

(Meydenbaur's) 185, 186 

(Ney's) 231-234 

(Pollack's) 241-242 

(U. S. Coast and Geodetic Survey's) 196-200 

(Vallot's) : . . . . 253-262 

(Vogel's) 184 

(Werner's) 184 

telescope (of Lechner) 244 

(of Paganini) 219, 220, 229 

(of Starke and Kammerer) 245, 249 

Canadian Department of the Interior 29, 31, 132 

Government Printing Office 132 

Pacific RaUroad 29 

phototopographic method 132 

outfit 193 

surveys 340 

Topographic Surveys Office 135 

Carbonate of soda 358, 359, 361, 363-365, 379 

ammonia 352 

potassium 361, 362, 365 

Carbonic acid 353 

Carbutt's color-screen 340 

orthochromatic films 255 

Casella, L. P 262 

Caucasus 209 

Caustic soda 361, 368 

Cazes, L 11 

Center, geometrical 165 

Centered lenses 164, 165 

Central projection 167 

Centre linead 290 

Chamounix base 262 

Chemical focus 180 

Chevallier, A 8, 15, 28 

Chicago Exposition 8) 29 

Chloroform 375 



414 INDEX. 

PAGE 

Chloride of ammonium 370 

gold 382-384 

and sodium 3S4 

platinum 373 

potassium 372, 375 

silver iii, iv, 372 

sodium 381, 383, 38s 

Chloroplatinite of potassium 383 

Chromatic aberration (secondary) 178-181 

Chrome alum 367 

Chromium salts 372 

Cian del Lei 89, ic6 

Citric acid 357, 383 

Clearing solution 357, 366, 369 

Cloud hoods 390 

Coast pilot work, Italian 223 

Cogne Valley 25 

CoUinear lens (Voigtlaender's) , 183 

Collodion iodide iv 

Color-screens 187, 198, 205, 339-341 

(of Bausch and Lomb) 341 

(Carbutt's) 340 

(Dallmeyer's) 341 

(orange) 187, 340 

(y«"o^) 198, 340, 341 

Colson, Capt. R 1 1, 44 

Combined toning- and fixing-bath 379, 383, 384, 386 

Commissioners, Alaskan Boundary 31^ 32, 135 jgy 

Comparative exposures ,4, 

<iiagram 345,346 

table 345 

light values ,43 

Compass, azimuth 203, 223, 227, 266 

'I'^l 226, 244 

Dixey 203 

Schmalkalder. 20^ 

Concave-convex lens jgr 

Concluded points jqg 

Congress, IX. Geographical, Vienna 26 

Conjugate foci 171-174 

planes 171-173 



INDEX. 415 

PAGE 

Conjugate points 174 

Conservatoire des Arts et Metiers, Paris 8 

Constant focal length 183, 185-186, 204 

Contouring, iconometric 126, 151, 157 

Convex-concave lens ; 165 

Copal 37S 

Copper sulphate 371 

Cross levels 233 

Cross-wires 213, 214, 221, 229, 265, 266 

Crown glass 180 

Crystallization of films 368 

water 350 

Curvature 214, 399 

Cyanide of potassium 371 

silver 370 

Cyanine 339, 340 

Cylindrograph, topographic 271-274 



D. 

D'Abbadie 8 

Daguerre iv 

Daguerreotype iv 

Dallmeyer, T. R 403-407 

Dallmeyer's rapid rectilinear lens 183 

wide-angle lens 340 

Danish plane-table alidade 242 

Dark-room 377 

Dark -tent v 

Daussy ". 7, 10 

Davy iii 

Definition, error in 215, 217 

of bisected points 229 

De Geer, Prof. G 22 

De Iriarte, C 28 

De La Fuye, A 11 

Denison's orange tissue 349 

Denkmaeler Archiv 14 

Density of a negative 336 

Department of the Interior, Canadian 29, 31, 132 

Derogy, G 402 



'4l6 " INDEX. 



Developer 353~3(>6 

amidol , 366 

bromo-hydrochinon 364 

eikonogen 366 

eiko-cvim-hydro 365 

ferrous oxalate 356 

hydrochinon 361, 362 

metol 360 

(bicarbonate of) soda 360 

hydrochinon 363 

pyro 358 

Developing agents 350 

Deville, Capt. E viii, 11, 29, 30, 33, 132, 135, 136, 186, 197, 199, 228, 

308, 342, 353> 355, 394, 398, 401 

Deville's photographic surveying method 132 

De Zea, Lieut.-Col. Dom Pedro 28 

Diagram of comparative exposures 345, 346 

Dial compass 226, 244 

Diaphragms of cameras 186, 259, 342 

lenses 166, 167, 176, 177, 201-205, 212, 215, 344 

Diffused light 337 

Dispersions 178, 179 

Distance line 47, 51-55, 190 

Distance between camera stations 399 

Distilled water 352 

Distortion, barrel-shape 163, 177 

optical 163 

pin-cushion 163, 177 

radial 164 

tangential 164 

in backed prints 402 

negatives 401 

prints 401 

produced by diaphragms ijy 

Dixey compass 20, 

Djamaht j c 

Doergens, Dr. R viii, 12, 13, 15, 17, 184 

Doergens' camera jg4 

Dolezal, Prof. E j-n 22 

DoUand j-g 

Dominion land surveyors (Canadian) jcc 



INDEX. 417 

PAGE 

Dom Panunce 6 

Douglas-Archibald, E 114 

Driffield, V. C : . 35 j 

Drouin, F 11 

Drying negatives 369 

Dry plates 335, 336, 339 

Dry-plate process v 

Dubosque 8 

Ducretet, E 244 

Duffield, Gen. W. W 32 

Dumas, FI 11 



E. 

Eccentric center of gravity 229 

Eccentric telescope 238 

Eclimetre 260 

Eckhohn, Nils 22 

Eder, Dr. J. W 18, 19, 340, 355 

Edwards, B. J 33 

Edwards' isochromatic plates 340 

Effect of color-rays, qualitative 336 

quantitative 336 

light, actinic 338 

optical 338 

Eikonogen 264, 265 

developer 366 

Eiko-cum -hydro developer 365 

Elements of a photographic perspective T19 

Elevations, determination of 80, 88, 95, 97, 119, 125, 132, 145, 147 

Emergent ray 166, 176 

Emery 7 

Engineering surveys vii 

Eosine ■■■■339 

Eritrea 88, 200 

Error in definition 215, 317 

iconometric angular measures 393^395 

stereoscopic base 328 

stereotelemeter readings 315 

swing of stereoscopic plates 328 

Erythrocine 339 



41 8 INDEX. 

PARE 

Exposition, Paris '. . 7 

Chicago 8, 29 

Exposure, length of 177, 336 

correctly timed. 336 

overtimed 336 

undertimed 336 

Exposures, comparative 343-346 

test 346-348 

Eyepiece, Ramsden 220, 221, 239 

F. 

Fahrlaender, Col 23 

Fa verges 7 

Fenner 26 

Ferrero, Gen . 24 

Ferricyanide of potassium 362, 377 

Ferrous oxalate 353 

Films, Carbutt's 258 

crystallized 365 

Finsterwalder, Dr. S 15, 17, 20, 27, 157, 227, 228, 245, 400 

Finsterwalder's phototheodolite 227-230 

Five-point problem 55-59 

Fixed focus 183, 185, 186 

Fixing-bath I ; 365-368, 377, 384 

Fixing and toning-baths 379, 382-384, 386 

prints 381 

Flare spots 166 

Flexible prints ,g2 

Flmt glass 180, 181 

Focal length of pin-hole camera ^ , 

constant 183^ 185, 186 

to compute 89, 90, 92, 98, 102, 104, 107, 110, 112, 128, 129, 

215, 217, 228, 233, 236 

lengths 161, 170-175, 183-186, 189, 191 

planes 168-175 

scales 209, 217, 220, 247 

variation ^ j8o 

Foci, conjugate 171-174 

f ocu=, chemical ; jg^ 

optical j8o 



INDEX. 419 

PAGE 

Focus, principal 168 

visual ' 180 

Foerster, Dr 15 

Fourcade, T ■ 8 

Fraunhofer lines 338, 341 

Freiburg 14 

French ministry of culture 184 

Fribourg, Commandant 8 

frilling 351, 367, 369 

Fritsch, G 19 

Front planes 37, 40, 31° 

Fuming-box 378 

G. 

Galileo 25, 224 

Garibaldi 7 

Gassr Dachel-. 14 

Gautier-Prandl prism 313 

Gay Lussac iv 

Gelatine emulsion v 

General staff, Prussian • 12 

Swedish 22 

Swiss 23 

Genie corps, French 258, 271 

Geographic latitude, photographic 224, 225 

Geological surveys vii 

Geometric center of lens 165 

Gimbal support for camera 225 

Girard, J 11, 16 

Glass, crown 180 

flint ' 180, 181 

Jena ' 178 

Gleaves, Albert 33 

Glycerine 382 

Goerz, anastigmat of 183, 187, 199, 201, 227, 229, 239 

Gold 385 

bank -envelope paper , . . . 349 

solution 379^389 

Golfarelli, Prof. 1 27 

Goulier, Col. Th 254, 260 



420 INDEX. 

PAGE 

Gran Sasso 24 

Graphic hypsometer 284 

protractor 275-278 

sector 278-283 

Grenoble 9 

Grimsimski, R 20 

Grousillier, H ■ 309 

Guenther, Oscar 238 

Guillemont 9, 402 

Gum guaiacum 188 



H. 

Hafferl, F 16, 19, 26, 107-110 

Hagstrom, H. L 22 

Halation i56, 342 

Halo 166 

• Haloid, silver 335-354 

Hamberg, Dr. A 22 

Hanot, Alfred n 

Hardener and short-stop 385 

Hartl, Lieut 16 

Hauck, Dr. G 15, 17, 62, 63, 80, 81, 139, 388 

Hacker, Dr. 20 

Heights, scale of 14^ 

Heine, L 20 

Heliography iv 

Heller, Prof. J 17 

Helmholtz ,12 

Herring, E 20 

Hess, Dr. H j e 

Heun, Dr. K 21 

High School, Technical, Berlin 14 g^ 

Prague 16 

Hildebrandson, H. H 21 22 

Hintze j , 

Hondaide'. p^ 402 

Hondaille, Capt ' j j 

Hood for lens igg^ jp^ 

Horn, Photographische Mittheilungen j , 

Horizon, artificial jgg jg.^ 



INDEX. 421 



PAGE 



Horizon line 185-193, 197, 206 

Horizontal angles 230 

contours 126, 151, 157 

intersections 115, 117, 124, 131, 140, 149 

Huebl, Baron von 243, 250 

plane-table photogranuneter of. 250-253 

Hurler, F 353 

Hydrochinon 361-365 

developer 361, 362 

Hydrographic survejrs vii 

Hydrometer tests 350 

Hyposulphite of sodium 357, 365, 367, 370, 373, 382, 383, 385 

tests 368 



L 

Iconometers 275 

Iconometric angular errors 393~39S 

contouring 126, 151, 157 

plotting 47, 49, 126, 151,157, 215, 307 

range 396-398 

Iconometiy 47, 49 

Identification of pictured points 138, 139 

Image plane 170, 202 

plate 215 

stereoscopic 319 

Imfeld 19 

Incident ray 166-176 

Inclined photographs 76, 78-80 

Index-mark of stereocomparator. 319, 321, 324, 326 

Index, refractive 159, 161, 178, 179 

Institute, Military Geographic, Italy 112 

Vienna 15 

Photogrammetric, Berlin 14 

Polytechnic, Milan 24 

Intensification 369, 370 

Intensity of light 205, 346 

Interocular distance 309, 317 

Intersections, horizontal 115, 117, 124, 131, 140, 149 

vertical 134, 142, 144. 3^9 

Iodide of potassium 370 



422 INDEX, 

PAGE 

Iodine, tipcture of 369 

Iron developer 356 

perchloride ._ 373 

sulphate 355-357 

Isochromatic dry plates 339 

Italian Military Geographic Institute 112 

phototopographic method 88 



J. 

Jarret 9, 402 

Javary, Capt 7, 10 

Jena. /. 178, 309 

glass 258 

Jesse, 21 

Jewell, L. E ._ 362 

Johnson 13 

Jordan, Dr. W 14, 16, 83-86, 394 

Jouart, A 8, 10 

Jungfrau. 23, 238 

K. 

Kennett v 

Kernel planes 64 

points 63-68, 304 

Kempunkte 63, 64 

Kerschensteiner, Dr 15 

Kinberg, Ma'or H 22 

King, Dr. W. F viii, 31, 135 

Klot2,O.J X9,33 

Kobsa, R 18 

Konistka, Dr. Karl jr 

Koppe, Dr. C iS, 17, 234, 238 

Krauss 258 

Krifka, Col. Otto 27 

L. 

Lacombe g, 402 

Lambert, F. C 43^ 44 



INDEX. 423 

PAGE 

Lambert, J. H 5, 35 

Lampblack 188 

Laska, W .' 20 

Latent image 334, 336, 343 

Latitude, photographic determination of 224, 225 

Laugier 7,10 

Laussedat, Col. A v, \iii, 4, 6, 12, 15, 27, 28, 33, 34, i2j, 130, 253, 298, 308 

Leather plate-holder 228 

Le Blanc, Capt 6 

Le Bon, Dr. G 10, 86-88, 184 

Le Bon's camera 184 

Lechner, R 250 

Lechner's camera 184, 244 

Le Conte, Prof 35 

Le Comu, J 11 

Le Gros, Commandant v, 156 

Lejeune, L 244 

Length, focal, computation of 89-92, 98, 102, 104, 107, no, 112, 128, 129 

Length of exposure 117 

stereophotogrammetric base 326 

Lens, anastigmatic, Zeiss' 183, 187, 199, 201 

aplanatic, Steinheil's 183 

antiplanatic, Steinheil's 212 

biconcave 165-174 

biconvex 165-1 73 

concave-convex 165 

convex-concave 165 

coUinear, ^'oigtlaende^'s 183 

combination 166 

diaphragms 166-177, 201-212, 215, 344 

double anastigmat of Goerz 183 

doublet 2It 

hood 194 

negative 165, 173, 174, 405-406 

pantoscopic, of E. Busch 183 

periscopic 165 

positive 165-170 

principal axis of 165 

rapid rectilinear, Dallmeyer's 183 

stops 166-177, 201-212, 215, 344 

vertices of a 165 



424 INDEX. 

PAGE 

Lenses 159, 161-163 

centered - 164 

Liesegang, Prof. R. E 371 

Light intensity 265 

Light-rays, sources of 337 

Light intensity, table 346 

Light values, comparative 343 

Lime 352 

Line, distance •. 47; Si; 55 

front > 38 

horizon 5i-S4 

principal 38, 51-54 

vanishing 41 

Literature, Austrian 16 

English -^^ 

French 10 

German 16 

Italian 27 

Spanish 28 

Swedish 22 

Swiss 23 

Loehr, Max 11 

Long-distance photography 402 

Lowison, Major 22 

Lumifere's orthochromatic plates 255 

Lundal, A 22 

Lunette d'Etat Major ; 402 

Lynn Canal 22 



M. 

MacArthur, J. J 29, 31, 32 

Maddox, Dr v 

Magnesia ,25 

Magnetic 'azimuth camera 226 

Magnification of the stereotelemeter oj, 

Mallmann ,40 

Mandl, Julius 21 

Manzi, Michele 24 

Marselli Capt. Carlo 2^ 

Martens g 



INDEX. 425 

PACE 

Matthieu 8, 402 

Maurer, ]M 16 

Measuring distorted negatives 238 

Mendenhall, Dr. T. C 32 

Meteorological observations vii 

Method, phototopographic 82 

Finsterwalder's 157 

French 121, 130 

German 83-86, 128 

HafFerl's 107, no 

Hauck's 63, 80, 81, 139 

Italian 88-115 

Jordan's 83-86 

Laussedat's 121, 130 

Le Bon's 86-88 

Legros' 156 

Meydenbaur's 128 

Paganini's 88-151 

of squares 72, 294 

Methods, topographic 2 

Metol developer 359, 360, 363, 373 

hydrochinon developer 363 

soda developer 360 

Meydenbaur, Dr. A 12, 14-16, 185, 186, 392 

camera 185, 186 

Miethe, Dr. A 19, 403, 404 

Mikiewicz, Lieut. L 16, 17 

Military Academy, U. S., West Point 4 

Geographic Institute of Italy 112 

Austria 15 

Ministry of Culture, French 184 

Moessard, Commandant P 10, 163, 271-274 

Monet, E 11 

Mont Blanc 9, 253, 258, 261 

Mont Cenis 24 

Mueller, Wilhelm 250 

Muriatic acid. .* 353, 385 



426 INDEX. 



N. 

PAGE 

Nadar 8 

Nathorst, Prof. A. G 22 

Navarro, L •. 28 

Negative ' 45 

lens 173, 174, 405. 406 

Negatives, to dry ." 369 

measure distorted 238 

intensify 369, 370 

reduce 373 

varnish 375 

stained 370, 373 

distortion in 401 

Neuhauss, Dr 228 

Ney's phototheodolite 231, 234 

Niepce, J.N iv, 6 

Nitrate of ammonia 374 

lead 383 

silver iv, 371 

sodium 374 

Nitric acid 385 

Nodal planes 165, 170, 174, 175 

points 16J-168 

Nonactinic paper 349 

Nonhalation plates 342 



O. 

Oasis Dachel 14 

Obemetter 340 

Objective scale 201, 209, 217-220, 247 

Observatory at Upsala, meteorological 21 

Observing angles from negatives 236-241 

Ocular lens of telescopic camera 219 

Oetz Valley 15 

Olson, K. P 22 

Optical axis 202-206 

center 165, 166 

distortion 163 

focus 180 



INDEX. 427 



PAGE 



Optical lens 161. 162 

sensitizers 339 

Oreo Valley 25 

Orienting picture traces. . . 47-49, 51-54, 62, 78, 92-94, 99, 124, 130, 137, 226 

Orthochromatic plates 339 

Orthogonal projection 210 

Ott, Max 227 

Ottawa 31, 132, 195 

Overexposure 348, 356, 363, 364 

Oxalate of iron 353 

potash 355 

Oxalic acid 352 



Pacific Ocean. 196 

Packing-case, light-tight 235 

Paganini, L. P viii, 3, 22, 25, 26, 88, 200-209, 211-227, 244 

Paga ini's camera 200-209, 212 

Panoramic cameras 270-274 

Pantograph arms 306 

Parallax 311, 318, 321, 326, 330 

Paris, map of 7 

Pate, E 8, 10 

Peep-sights 260 

Perchloride of iron 373 

Period of reversal 336 

Permanganate of potash 368 

Perspective axis 65, 67 

elements 35) 37, 1 19 

photographic 36-42 

Perspectographs 275, 298-302, 303-307 

Perspectometer, Deville's 294-297 

Phosphate of soda 352, 375 

Photogenic drawings iv 

Photograrameters 182 

Photog ammetric plane table of von Huebl 250-252 

Photogrammetry and phototopography 13, 387 

Photograph board, Deville's 137, 138, 148, 290-294 

protractor 154 

Photographic azimuth compass •" 223-227 



428 INDEX, 

PAGE 

Photographic determination of magnetic azimuth 224 225 

outfit, Canadian 193 

perspectives 36-42 

plates, inclined 76 78-80 

printing 376 

Photometrography 13 

Phototachfomfetre 253 

Phototheodolite of Bridges-Lee 262-269 

Capt. E. Deville 186-188 

Dr. S. Finsterwaldei 227-230 

Dr. C. Koppe 234-241 

Col. A. Laussedat 244 

R. Lechner 243 

L. P. Paganini 211, 219 

O. Ney 231-234 

V. Pollack 241-242 

Pollack and Haflferl 242-243 

Starke and Kammerer 245-250 

the U. S. Coast and Geodetic Survey 196-200 

Vallot Frferes • 253-262 

Phototopographic plane table of Chevallier 270 

Picture traces 47-49; 5I-S4, 6'2, 78, 92-94, 99, 124, 130, 137, 327 

Pietsch, Dr 1 14-16 

Pie-y-Allu^, Don Juan 28 

Pin-cushion distortion 177 

Pin-hole-camera constants 44 

exposures 43 

cameras 42 

diameter 42 

photography 42 

Pizzighelli, Major .••••. 16, 17 

Planchette photographique 8, 28 270 

Plane, conjugate 171-173 

datum oy 

focal 168-175 

^■■o^t 37, 40, 310 

ground ,y 

horizon ,y 

i™age lyo, 202 

kernel 63-68 

^°'^^ 165, 170, 174, 17s 



INDEX. 429 

PAGE 

Plane, principal 37, 38 

table photogrammeter of von Huebl 250-253 

visual 39 

Plate carrier 186 

holder 186, 193, 194 

Plates, overexposed 348, 356, 363, 364 

size of, Bridges-Lee's '. 268 

Finsterwalder's 227 

French 245 

Huebl's 253 

Italian 201, 202, 222, 225, 226 

Meydenbaur's , 185 

Ney's 233 

U. S. Coast and Geodetic Survej''s 199 

Vallot's 25s 

stereoscopic ." 31 

test 346-348 ' 

underexposed 349, ^6^ 

verticality of 100-102 

Platino-bromide paper 136, 378, 401 

Platinum 379, 383 

Plotting contours 126, 151, 157 

iconometric , 49, 115, 117, 124, 131, 140, 149, 215 

picture-traces 47-51. 54, 62, 78, 92-94, 99, 124, 130, 137, 327 

shore lines 126, 151, 157 

stations 55 

Pneumatic shutter 226, 234 

Point, distance 39 

kernel 63-68 

nodal 36-38, 165-168 

principal 38, 47, 51-55, 190, 202 

reference .■ 93, 134, 139, 140, 152 

vanishing 39, 41 

Pointer, station , 60 

Points, concluded 196 

conjugate 174 

iconometric plotting of 115, 117, 124, 131, 140, 149 

identification of 138, 139 

trigonometric 218, 229 

Polar iconometric method 387, 391 

precision of 39^ 



43° INDEX. 

PARE 

Pollack, V 16, i8, 27, 243 

Porro, Prof 24, 223, 232, 309: 

Porta, G. della 6 

Portland Canal 32 

Positive lens. 170-173 

Positives 45; 

Potassium bromide 355, 356, 359, 361, 364, 365, 37r 

chloride 372^ 

cyanide 371 

Pothenot 60 

Prague 15 

Precision of polar-iconometric method 391 

Pribilof Islands 32- 

Principal axis of a lens 165 

foci of a lens 168 

line of a perspective 38, 51-54, 185-193, 197, 206 

planes '. 168 

point 38, 47, 51-55. 19°, 2°^ 

ray 3& 

Printing by development 377 

frame 376 

out papers 376, 377 

photographic 376 

Prints, distortion in 401 

distorted through backing 402 

flexible 382 

"splotchy" 385 

Problem, five-point 55-59 

three-point S9-62 

Protractor, photograph , 154 

three-arm 60- 

Prussian Topographic Bureau 319 

Prussiate of potash 352 

Pujo, Th 8 

Pulfrich, Dr. C 20, 21, 308, 314 

Projection, central 36, 167 

orthogonal 36, 40, 48, 210 

outward 36 

Punta Bivula 113 

dell' Erbetet 114 

dell' Invergnan 114 



INDEX. 



431 



PAGE 

Punta del Lei 89, 106 

di Breuil 114 

di Nomenon ; 95, 967 100-106, 114 

di Toss 114 

Gran Paradiso 113 

Percia 95, 99, 100, 114 

Rouletta 95, 96, 99, 100, 114 

Ruja 91, 103, 105, 106 

Pyro developer. 358 

Pyrogallic acid ("Pyro") 358 

Pyro-ammonia 373 

-metol 373 

-soda 373 



Q- 
Quidde ^3 

'R. 

Radial method 387, 391 

precision of 391 

Ramsden eyepiece 26, 220, 221, 239 

Randhagen, F 236 

Range-finder, stereoscopic 309 

Range of stereoscopic vision 312 

Rapidity of lens 176, 177 

Rapid rectilinear lens, Dallmeyer's 183 

Ray, emergent 166-176 

incident 166-176 

Ray-filters ■' 187, 198, 205, 339-341 

Ray, principal 38, 51-54, 185-193, 197, 206 

Reducer 373 

Reed, Lieut. Henry A 4 

Reference points 93, 134, i39> 140, 152, i-9S 

Refraction 160, 178, 179, 214, 338, 399 

Refractive index 159, 161, 167, 170, 179, 180, 188 

Regnault 294 

Reichel 232 

Relief effect 310 

Retina of the eye 180, 219 



432 INDEX. 

FAOB 

Restrainer 356, 361 

Rhemes 95, 113, "4 

Rhodamine 339, 340 

Rholf, G 14 

Ringertz, Major N. C 22 

Ritter, Herman 17 

Rocky Mountain Park. . 29, 30 

Rockwood-Schallenberger panoramic camera 271 

Roscoe 346 

Rosen, Prof 22 

Rosenmund, M 23 

Ross, Thos 28 

Rousson, H '. 11 

Rubber 375 

Ruby light 349 

S. 

Sal ammoniac 375 

Salmairaghi 24 

Salt (chloride of sodium) iv, 381, 383, 385 

Saltpeter 375 

Santa Cruz Island S 

Scale, objective 209, 217, 220, 247 

of heigKts 147 

stereotelemeter 317 

transverse, aerial 310 

vanishing 74, 75 

Schallenberger 271 

Scheele ■ iii 

Schell, Prof. 16 

Schepp, A 25 

Schmalkalder compass 203 

Schott, Dr. A 178 

Schiffner, Prof. F 16, 19, 20, 56, 303 

Schreiner 13 

Schroeder, Dr 15, 17, 18 

Schroeder & Co 298 

Schumann 340 

Schwassmann, A 20 

Scolik 340 



INDEX. 433 

PAGE 

Scott's exposure table 34S 

Screens, color 187, 198, 205 

test 163, 164 

Secondary chromatic aberration 178 

Sector, graphic 278-283 

ordinary 146 

Seeing, stereoscopically 314 

visual 35 

Selecting camera stations 388 

Seliger, P 319 

Sensitizer, optical 339 

Sensitized albumen paper 378 

Sensitometer number 205, ^36 

Settore Grafico 278-283 

Shipping-box for plates 195 

Shore line, to plot 68-71 

Short-stop and hardener 385 

Silver bromide v 

chloride of iii, iv, 372 

cyanide of 37° 

haloids, reduction of 335, 354 

nitrate iv, 371 

printing-paper 378 

Simon, S 23 

Size of photographic dry plates, Bridges-Lee's 268 

Finsterwalder's 227 

French 245 

Huebl's - 253 

Italian 201, 202, 222, 225, 226 

Meydenbaur's 185 

Ney's 233 

U. S. Coast and Geodetic Survey's 199 

Vallot's 255 

Snellius 60 

Society for Natural Research, Munich 309 

Sodium carbonate 35^, 359, 361, S^SS^S 

Sodium phosphate 35^ 

Southeast Alaska i97, 34° 

Spectrum, solar i", 337-339, 34i 

Speed of photographic plates 336 

Spirit-levels for camera 184-192, 197, 203, 205 



434 INDEX. 

PAGE 

Spitaler, Dr. A 403 

Spitzbergen 22 

Splotchy prints 385 

Sprung, A 20 

Squadro Grafico 284-290 

Squares, method of 72 

Stained negatives. . , 370, 373 

Stanley ^^ 

Starch 368 

Starke, G 19 

Starke and Kammerer 254, 249 

Station, foot of 38 

plotting SS 

pointer 60 

Stegemann's camera 183 

Steiner, Prof. F 16, 18, 54, 55 

Steinern, Baron von 209 

Steinheil, Dr 183, 212, 222, 403, 404 

Steinheil's antiplanatic lens 212 

aplanatic lens 183, 222 

Stereocomparator 308 

Stereophotogrammetry 308, 318 

Stereoplanigraphs 308 

Stereoscopic power 309 

surveying 308 

telemeter ■ 308, 318 

Stolze, Dr iS, 17 

Strachy, R 33 

Strassburg -, . . j^ 

Sugar '. 383 

of lead (acetate of lead) 383-384 

Sulphate of copper oyi 

'''°° 355-357 

sodium ,yj 

Sulphite of sodium 358-367, 382, 385 

Sulphocyanate of potassium ,^4 

Sulphocyanide of ammonium ,g2^ ,8, 

Sulphuric acid 355, 361^ ^67 

Superintendent U. S. Coast and Geodetic Survey, Report of 34 

Surveying-cameras, requirements 183 

Surveyor-General Dominion lands of Canada 132 



INDEX. _ 435 

PAGE 

Surveying, phototopographic, in Alaska 29 

Austria 15 

Canada 29 

France 5-9 

Germany 12 

Italy 24 

Spain 27 

Sweden 21 

Switzerland 23 

Survey's Office, Canadian Topographic 135 

Sutton 28 

Swiss Topographic Bureau 236 



T. 

Table salt (chloride of sodium) iv, 381, 383, 385 

Tachfomfetre 253 

Talbot, Fox iv 

-Talbotype iv 

Tannin solution 353 

Tasmania 5 

Technical High School, Berlin 14, 63, 302 

Braunschweig 234 

Teeth, camera 214 

Telemeter, stereoscopic 313, 318 

errors in reading the stereoscopic 315 

Teleobjective 259, 402, 403-405 

Telephotography 402 

Telescope, "broken" 260 

Telescopic camera, Italian 220 

Lechner's 244 

ocular lens 219 

of Starke and Kammeirer 245, 249 

diaphragm 219, 229 

Temperature for toning-bath 380, 385 

Test exposures 346-348 

plates for stereoscopic vision 314 

photographic 334, 346-348 

screens, photographic 163, 164 

Testing water 352 

Tests for "hypo" 368 



43^ INDEX. 

PACK 

Tetrachlor-tetraethyl-rhodamine-clilorhydrate 340 

Theodolite of Col. Goulier 254, 260 

Three-arm protractor 6<> 

Three-point problem 59> 62 

Tiflis 209 

Tincture of iodine 369 

Tissandier, G 12 

Toning process 379-381 

Toning- and fixing-baths, combined 379, 383, 384, 386 

separate (or plain) 379, 382, 383 

Topographic Bureau of Switzerland 236 

cylindrograph, Moessard's 271-274 

Surveys Branch of Canada 135 

Topography, definition of 1 

Tourniquet 163 

Transit 187, 188, 195, 19& 

theodolite 231 

Transverse scale 310 

Trial plates 334, 346-348 

Tribrach (trian'gular instrument base) 187, 198, 263^ 

Trikolograph, Dr. Hauck's 302-306 

Trilinear systems 63 

Tripods for cameras 185, 187, 193, 197, 204, 211, 222 

stereotelemeters 313, 



U. 

Underdevelopment , ,40. 

Underexposed plates ^^n 36, 

Uniform diaphragm system ,^ 

U. S. Alaskan Boundary Commissioner loy 

U. S. Coast and Geodetic Survey 25, 3i, 32. 34, 185, 196-198, 243, 

340, 342 
Upsala Meteorological Observatory 21 



V. 
Valenta, Dr. E ^^ 

™'H ■■■■■■■■9,253 

Vallot,; 5_,5^ 

Valsavaranche Valley 25, 91, 113, 114 



INDEX. 437 

PAGE 

Valsoana Valley 25, 

Van Diemensland (Tasmania) 5 

Vanishing points 39-41 

scale 74^75 

Variation, focal 180 

Varnish for negatives 375 , 

Vautier-Dufour, A 12 

Verein, Oesterreichischer Ingenieur und Architecten 27 

Vemach glacier 15 

Verner, C. W 33 

Vertical angles 230- 

intersections 134, 142, 144, 389. 

Verticality of plates 100-102 

Vertices of a lens 165 

Vienna 16 

Vision, range of stereoscopic 31a 

Visual focus i8a 

Vogel, Dr. H. C 401 

Vogel, Dr. H. W 13, 15, 17, 184 

Vogel's camera 184 

Voigtlaender's collinear lens 183 

von Alster, Gen 12 

von Guttenberg, A. K 20 

von Huebl, Baron 16, 19, 20 

von Moltke, Count 12 

von Steinem, Baron 209 

von Stemeck, Col 26 

von Ziegler, Ch 12 

Volkmer. 19 

W. 

Wallon, E 12 

Wang, F 19 

War, Franco-Prussian 12, 13 

Water tests ■ 352 

Wedgewood iii 

Weight of phototheodolite, Finsterwalder's , 227 

Laussedat's 245 

Paganini's 200 

Vallot's 261 



438 



INDEX. 



PAGB 

Weight of phototopographic magazine camera, Meydenbaur's. 185 

photogrammetric plane table, von Huebl's 253 

stereotelemeters 313 

Wenz, E 12 

Werner 343 

A\ erner's camera 184 

Westman, J 22 

Wet-plate process iv, v 

Wheatstone stereoscope 308 

Wliipple, G. M 33 

Wide angle lens 340 

A\'iganowsky 8 

Woodbury antipyr. 374 

Wollaston 6, 298 



Y. 

Yellow color-screens 198, 340, 341 

stains 31°> 373 

Z, 

Zeiss' anastigmat lens. 164, 183, 187, 199, 201, 222, 226, 227, 242, 243, 247, 258 

Zeiss, Carl 178, 309 

Zeitschrift fuer Instrumentenkunde 230 



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PLATE XCIX 





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C.ZEISS, JENA 



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